feat: new BP_GO_WORKDIR variable

[gauron99]

Summary

Introduce new BP_GO_WORKDIR which can be set as a relative subpath of the whole working directory (--path) as a path where go build will be build from. Meaning effectively cd into subdir && go build... happens.

Use Cases

Use case for mono repos and non-standard repos. The current implementation lacks option for main package to be different to the root of the project.

If a repo has non-standard dependency -- eg. when root is not the "main" package the build fails -- it expects the root to be the main package and derives its dependency tree in standard manner: from root to subdirs. If a repo uses non-standard go dependency tree it needs to build in the main package which means, in this case, from sub directory and this is what BP_GO_WORKDIR aims to enable; differentiate context directory (--path when pack building) and go workdir directory.

fixes #879

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[linux-foundation-easycla]

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• ✅ login: gauron99 / name: David Fridrich (cf17397)