Windows port of bb_runner and bb_worker

[mou-hao]

This change intends to port bb_runner and bb_worker to Windows and is part of #7. Please review this in conjuncion with buildbarn/bb-storage#129.

Key changes include:

• Putting go_dependencies() before go_rules_dependencies() for the same reason detailed in Windows port fixes and improvements bb-storage#129.
• Adding Windows local_runner and, because it shares a lot of code with the unix runner, I refactored the code to have the two share a common local_runner.go.
• Ignoring the platform property bazel-differentiate-workspace-cache when executing. The property is added as a hack for CPP builds on Windows. It is a hack to differentiate the hash keys depending on the workspace and is not intended to mean much to the remote executor: Remote: Use execRoot as input root and do NOT set working directory by default. bazelbuild/bazel#13339

I will create a draft PR first and, if and when buildbarn/bb-storage#129 looks good and can be merged, I will update the dependency here.

Cheers,
mh

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[EdSchouten]

Looking forward to seeing this land!

[mou-hao]

Should be good for another round of review. 😄

[EdSchouten]

Looking perfect. Thanks for your perseverance!

I've re-enabled CI again. It may be the case it complains about some missing docstrings on newly added public APIs? If so, could you address those? Will merge this change once CI is happy.

[mou-hao]

Looking perfect. Thanks for your perseverance!

I've re-enabled CI again. It may be the case it complains about some missing docstrings on newly added public APIs? If so, could you address those? Will merge this change once CI is happy.

Done.