cmd/go: invalidate test cache when -coverpkg sources change

[ryancurrah]

When using -coverpkg, test caching could return stale coverage data
if covered packages were modified but the test package itself was
unchanged. This occurred because the test cache only considered the
test package's dependencies, not the broader set of packages specified
by -coverpkg.

This change adds dependency tracking for covered packages and includes
their source file hashes in cache validation. When any file in a
covered package changes, the test cache is properly invalidated,
ensuring fresh coverage data is generated.

Fixes issue where coverage reports contained duplicate entries with
different line ranges after source modifications in covered packages.

Fixes #74873

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Jorropo:

Patch Set 1: Commit-Queue+1

(3 comments)

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Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-07-29T11:16:25Z","revision":"7c78557e0f55c0fbd57694af257866cdad171575"}

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Message from Jorropo:

Patch Set 1: -Commit-Queue

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Message from Go LUCI:

Patch Set 1:

This CL has passed the run

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Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result+1

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This PR (HEAD: f7cd72d) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/690775.

Important tips:

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[gopherbot]

Message from Ryan Currah:

Patch Set 2:

(3 comments)

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