Evaluate build tags as both true and false

[patrickmscott]

Gazelle already iterates over all supported os/arch combos to include go files. This is similar but expanded to custom tags. When adding a new build tag, we create a copy with the tag set to true and append to the original. This means that build tag evaluation will grow exponentially.

Fixes #1262

What type of PR is this?

Feature

What package or component does this PR mostly affect?

language/go

What does this PR do? Why is it needed?

It changes the behavior of build_tags to defer evaluation to build time. Instead of assuming the tag is always true, it treats the tag as both true and false when evaluating tags for a file.

Which issues(s) does this PR fix?

Fixes #1262

Other notes for review

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