feat(gazelle): Add directives for label format & normalisation

[wingsofovnia]

Adds new directives to alter default Gazelle label format to third-party dependencies useful for re-using Gazelle plugin with other rules, including rules_pycross.

Fixes #1939

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[aignas]

LGTM, only the snake_case to be modified to be more like pep503 left.

[aignas]

we also need to replace any occurrences of __ to _. This is done in our starlark code, so it is essentially the same as pep503 but with _, so doing the same regex as below would be ideal.

[wingsofovnia]

This piece was moved exactly from SanitizeDistribution to make sure nothing breaks existing behaviour. Could you please point to the Starlark code you had in mind?

I found this, and what my internal 🧠 interpreter tells me it escapes char by char, so distribution----with--dashes becomes distribution____with__dashes, not distribution_with_dashes (which Gazelle will expect if I updated this convention to replace "[-_.]+" with _ similarly to what pep503 does).

[aignas]

The rules_python//python/private:normalize_name.bzl is the code that I had in mind.

[wingsofovnia]

Thank you. I updated the code and actually found extra corner case where normalize_name.bz behaves differently than SnakeCaseLabelNormalizationType and the old code: prefix/trailing dashes. This has been fixed, the mapping updated, more tests based on normalize_name.bzl in/outs added.

[aignas]

Thank you!