Label crashes in BUILD files

[brandjon]

(Context: #12963)

When you try to call Label within a BUILD file, you're treated to an NPE crash.

Label is not ordinarily accessible in BUILD files, but you can bind it to a symbol with a different name, and load that symbol instead.

The crash is because Label attempts to retrieve the BazelModuleContext of the calling file, which doesn't exist for BUILD files.

This makes it harder to use an idiom whereby you pass Label objects as argument values to macros instead of strings. Such an idiom is needed if you want to lock in your own repo remapping instead of using the one in the repo of the macro that you're calling.

Before fixing this bug, we should reassess the nature of repository remapping. Even if this use case worked without crashing it would still be confusing to end users.

[github-actions]

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[github-actions]

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