Only invalidate repositories when an environment variables changes that they read

[lberki]

Description of the feature request:

Currently, external repositories are dirtied in Skyframe when any variable in --repo_env changes.

This means that there is a lot of unnecessary re-hashing to establish that the thus dirtied repositories are in fact up to date. This is because currently every repository depends on PRECOMPUTED:repo_env, which is just a bag of every such environment variable.

Ideally, repositories should only depend on the variables that they actually access.

There is some prior art in this area:

• c230e39 adds the ability to gather the accessed environment variables (not strictly necessary, but is a good pleprint)
• 80a4a14 makes repository rules not depend on PRECOMPUTED:repo_env if they don't read any environment variable. It's very crude, though.

Which category does this issue belong to?

No response

What underlying problem are you trying to solve with this feature?

No response

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

No response

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response

[555vedant]

Respected sir please can you assign this issuue to me ?

[lberki]

@555vedant are you volunteering to fix this issue? (Unless @Wyverald disagrees, I'd be delighted.)