[native_assets_cli] Nest c_compiler config

[dcharkes]

Changes the format for the YAML/JSON BuildConfig to have a nested c_compiler key. Example config:

c_compiler:
  ar: /tmp/MLTHBB/fake_ar
  cc: /tmp/MLTHBB/fake_clang
  env_script: /tmp/MLTHBB/fake_env_script.bat
  env_script_args: []
  ld: /tmp/MLTHBB/fake_ld
dependency_metadata:
  bar:
    key: value
  foo:
    a: 321
    z:
      - z
      - a
link_mode_preference: prefer-static
out_dir: /tmp/MLTHBB/out1/
package_root: /tmp/MLTHBB/
target: ios_arm64
target_ios_sdk: iphoneos
version: 1.0.0

This changes the environment variables to set if the compiler cannot be auto-detected on PATH or default install locations to:

• C_COMPILER__AR
• C_COMPILER__CC
• C_COMPILER__LD
• C_COMPILER__ENV_SCRIPT
• C_COMPILER__ENV_SCRIPT_ARGS

These will need to be updated in the Dart SDK: https://dart-review.googlesource.com/c/sdk/+/300221.

For reference internal design doc for the protocol.

Closes: #12

[coveralls]

coverage: 99.846% (+0.003%) from 99.843%
when pulling d71c2c1 on build-config-hierarchy
into e5cac81 on main.

[whesse]

I see the names of the environment variables have not been changed in the internal design doc. I assume you have buy-in from stakeholders on these new names.

I had to do a bunch of digging to see how the new environment values were being fetched in the non-test case: in package cli_config which is in the tools repository, with environment parser converting the env variable name to the key value.

[dcharkes]

Thanks @whesse! 🙏

I see the names of the environment variables have not been changed in the internal design doc. I assume you have buy-in from stakeholders on these new names.

This is addressing Martin's feedback on an earlier PR.

For now, we don't have to settle on the final API yet. That point comes when we start publishing these packages and stop using an experimental flag for the dartdev support. (Which at this point has not landed yet.)

I had to do a bunch of digging to see how the new environment values were being fetched in the non-test case: in package cli_config which is in the tools repository, with environment parser converting the env variable name to the key value.

Correct! 👌

(That was also why the non-test-case was/is failing on https://dart-review.googlesource.com/c/sdk/+/267340, I used a different env key for the script and script args.)