python: Implement has_pyX_only_sources computation for Starlark impl

This is a bit moot with Python 2 being dropped, but there's an assert for it
mixed in with some tests for other behavior, it shows up in PyInfo, and it's easy to compute with simple logic, so might as well.

Work towards #15897

PiperOrigin-RevId: 503465342
Change-Id: I3d5ab6fc1e8bd0dec25c2d71c94f1a59af8d4e38

src/main/starlark/builtins_bzl/common/python/common.bzl

@@ -342,6 +342,8 @@ def create_py_info(ctx, *, direct_sources, imports):
         necessary for deprecated extra actions support).
     """
     uses_shared_libraries = False
+    has_py2_only_sources = ctx.attr.srcs_version in ("PY2", "PY2ONLY")
+    has_py3_only_sources = ctx.attr.srcs_version in ("PY3", "PY3ONLY")
     transitive_sources_depsets = []  # list of depsets
     transitive_sources_files = []  # list of Files
     for target in ctx.attr.deps:
@@ -350,6 +352,8 @@ def create_py_info(ctx, *, direct_sources, imports):
             info = target[PyInfo]
             transitive_sources_depsets.append(info.transitive_sources)
             uses_shared_libraries = uses_shared_libraries or info.uses_shared_libraries
+            has_py2_only_sources = has_py2_only_sources or info.has_py2_only_sources
+            has_py3_only_sources = has_py3_only_sources or info.has_py3_only_sources
         else:
             # TODO(b/228692666): Remove this once non-PyInfo targets are no
             # longer supported in `deps`.
@@ -395,8 +399,8 @@ def create_py_info(ctx, *, direct_sources, imports):
         # NOTE: This isn't strictly correct, but with Python 2 gone,
         # the srcs_version logic is largely defunct, so shouldn't matter in
         # practice.
-        has_py2_only_sources = False,
-        has_py3_only_sources = False,
+        has_py2_only_sources = has_py2_only_sources,
+        has_py3_only_sources = has_py3_only_sources,
         uses_shared_libraries = uses_shared_libraries,
     )
     return py_info, deps_transitive_sources

src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java

@@ -97,8 +97,6 @@ public void librarySandwich() throws Exception {
         "    deps = [':pylib'],",
         "    imports = ['upperuserlib_path'],",
         ")");
-    // TODO(bazel-team): Implement support for `imports` attribute
-    setBuildLanguageOptions("--experimental_builtins_injection_override=-py_library");
     ConfiguredTarget target = getConfiguredTarget("//pkg:upperuserlib");
 
     PyInfo info = target.get(PyInfo.PROVIDER);