linkage: add --strict flag to detect opportunistic linkage

[carlocab]

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There was a previous discussion about making brew linkage --test fail
for unrequested dependencies (#9172). I'm not sure what the outcome of
that was, but it still seems like a good idea to try to help us find
cases of opportunistic linkage as they happen rather than when they
cause CI failures in another PR sometime later.

Let's do this by adding a --strict flag to brew linkage --test. My
intention is for brew linkage --test --strict failures to be warnings
rather than errors in CI, which should mitigate some of the concerns
about doing this that were raised in #9172.

[BrewTestBot]

Review period will end on 2021-11-22 at 10:19:40 UTC.

[MikeMcQuaid]

Makes sense to me.

I think we maybe used to do this (I can't remember) and the common issue was linking to recursive dependencies.

[carlocab]

Linking with recursive dependencies shouldn't cause a brew linkage --test --strict failure, since they should be here:

brew/Library/Homebrew/linkage_checker.rb

Line 42 in 27a7629

display_items "Indirect dependencies with linkage", @indirect_deps

If they do then that's a bug we can look at.

[MikeMcQuaid]

@carlocab Perfect ❤️

[BrewTestBot]

Review period ended.