support setting the base directory path of .files[].src to simplify

[suzuki-shunsuke]

Feature Overview

support setting the base directory path of .files[].src.

Why is the feature needed?

To simplify the configuration.

Does the feature include Breaking Changes?

No.

Example Code

https://github.com/aquaproj/aqua-registry/blob/fafe10db475840269ca00c934cd34fcae2c8c9ad/pkgs/WebAssembly/binaryen/registry.yaml#L19-L43

AS IS

files:
  - name: binaryen-unittests
    src: binaryen-{{.Version}}/bin/binaryen-unittests
  - name: wasm-as
    src: binaryen-{{.Version}}/bin/wasm-as
  - name: wasm-ctor-eval
    src: binaryen-{{.Version}}/bin/wasm-ctor-eval
  - name: wasm-dis
    src: binaryen-{{.Version}}/bin/wasm-dis
  - name: wasm-emscripten-finalize
    src: binaryen-{{.Version}}/bin/wasm-emscripten-finalize

TO BE

files_dir: binaryen-{{.Version}}/bin
files:
  - name: binaryen-unittests
  - name: wasm-as
  - name: wasm-ctor-eval
  - name: wasm-dis
  - name: wasm-emscripten-finalize

Reference