Determine if symlinks can be used with jinja templates

[parthea]

In PR #1999 , I attempted to use a symlink in jinja templates but encountered an error message jinja2.exceptions.TemplateNotFound: %namespace/%name/%version/%sub/services/%service/_shared_macros.j2

We should investigate if symlinks can be used. See #1999 (comment)

nox > Running session showcase_unit_alternative_templates-3.11
nox > Creating virtual environment (virtualenv) using python3.11 in .nox/showcase_unit_alternative_templates-3-11
nox > ----------------------------------------------------------------------
nox > Note: Showcase must be running for these tests to work.
nox > See https://github.com/googleapis/gapic-showcase
nox > ----------------------------------------------------------------------
nox > python -m pip install -e .
nox > python -m pip install grpcio-tools
nox > curl https://github.com/googleapis/gapic-showcase/releases/download/v0.35.0/gapic-showcase-0.35.0.desc -L --output /tmp/tmpcs2u3puq/showcase.desc
nox > curl https://github.com/googleapis/gapic-showcase/releases/download/v0.35.0/showcase_v1beta1.yaml -L --output /tmp/tmpcs2u3puq/showcase_v1beta1.yaml
nox > curl https://github.com/googleapis/gapic-showcase/releases/download/v0.35.0/showcase_grpc_service_config.json -L --output /tmp/tmpcs2u3puq/showcase_grpc_service_config.json
nox > python -m grpc_tools.protoc --experimental_allow_proto3_optional --descriptor_set_in=/tmp/tmpcs2u3puq/showcase.desc --python_gapic_opt=old-naming,python-gapic-templates=/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/ads-templates,transport=grpc+rest,service-yaml=/tmp/tmpcs2u3puq/showcase_v1beta1.yaml,retry-config=/tmp/tmpcs2u3puq/showcase_grpc_service_config.json --python_gapic_out=/tmp/tmpcs2u3puq google/showcase/v1beta1/echo.proto google/showcase/v1beta1/identity.proto google/showcase/v1beta1/messaging.proto
Traceback (most recent call last):
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates-3-11/bin/protoc-gen-python_gapic", line 33, in <module>
    sys.exit(load_entry_point('gapic-generator', 'console_scripts', 'protoc-gen-python_gapic')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates-3-11/lib/python3.11/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates-3-11/lib/python3.11/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates-3-11/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates-3-11/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/cli/generate.py", line 64, in generate
    res = generator.Generator(opts).get_response(api_schema, opts)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/generator/generator.py", line 122, in get_response
    self._render_template(
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/generator/generator.py", line 304, in _render_template
    self._get_file(
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/generator/generator.py", line 341, in _get_file
    self._env.get_template(template_name).render(
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates-3-11/lib/python3.11/site-packages/jinja2/environment.py", line 1304, in render
    self.environment.handle_exception()
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates-3-11/lib/python3.11/site-packages/jinja2/environment.py", line 939, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/client.py.j2", line 1, in top-level template code
    {% extends '_base.py.j2' %}
    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/ads-templates/_base.py.j2", line 5, in top-level template code
    {% block content %}
    ^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/client.py.j2", line 4, in block 'content'
    {% import "%namespace/%name/%version/%sub/services/%service/_shared_macros.j2" as shared_macros %}
    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates-3-11/lib/python3.11/site-packages/jinja2/loaders.py", line 207, in get_source
    raise TemplateNotFound(template)
jinja2.exceptions.TemplateNotFound: %namespace/%name/%version/%sub/services/%service/_shared_macros.j2
--python_gapic_out: protoc-gen-python_gapic: Plugin failed with status code 1.
nox > Command python -m grpc_tools.protoc --experimental_allow_proto3_optional --descriptor_set_in=/tmp/tmpcs2u3puq/showcase.desc --python_gapic_opt=old-naming,python-gapic-templates=/home/runner/work/gapic-generator-python/gapic-generator-python/gapic/ads-templates,transport=grpc+rest,service-yaml=/tmp/tmpcs2u3puq/showcase_v1beta1.yaml,retry-config=/tmp/tmpcs2u3puq/showcase_grpc_service_config.json --python_gapic_out=/tmp/tmpcs2u3puq google/showcase/v1beta1/echo.proto google/showcase/v1beta1/identity.proto google/showcase/v1beta1/messaging.proto failed with exit code 1
nox > Session showcase_unit_alternative_templates-3.11 failed.

https://github.com/googleapis/gapic-generator-python/actions/runs/8991765698/job/24700074783