feat(bzlmod): allow default_gazelle_attributes for go_deps extension

[tyler-french]

This PR creates a new tag class default_gazelle_attributes which allow the root module to provide default values for the gazelle attributes used to create the go_deps repos.

This includes the attributes: "build_file_generation", "build_extra_args", and "directives".

Default attributes will be applied to an attribute only if there is no gazelle_override value.

If default_gazelle_attributes is provided, it will override all path-specific overrides created in internal/bzlmod/default_gazelle_overrides.bzl, so users may need to reapply these with gazelle_overrides.

If a repo has either a gazelle_override or there is a default_gazelle_attributes, the "build_file_generation" attribute will be set to "on" unless explicitly set to a different value. This is because "build_extra_args", and "directives" anticipate build file generation being enabled.

If any value is provided for attrs in a gazelle_override, it will override (and not append to) any default attributes. Users must re-add any "directives" that were in default_gazelle_attributes

Additional refactoring was done to remove duplication of the handling of these attributes.

[tyler-french]

@fmeum Please do not merge, I have not yet tested this at Uber

[tyler-french]

@fmeum Please do not merge, I have not yet tested this at Uber

I tested this and it works as expected.

[fmeum]

The one thing I'm worried about is that the new default attributes are override by the per-module defaults provided by Gazelle. This makes adding a new default more breaking. But having it the other way round would remove most of the use of the Gazelle-provided defaults.
Not sure what's best yet and this may also change as we flip global defaults.

[tyler-french]

The one thing I'm worried about is that the new default attributes are override by the per-module defaults provided by Gazelle. This makes adding a new default more breaking. But having it the other way round would remove most of the use of the Gazelle-provided defaults. Not sure what's best yet and this may also change as we flip global defaults.

I agree, but I think this implementation provides the most user-friendly approach, given the following considerations:

1. Path-specific overrides that gazelle defines should only used for dependencies required to build gazelle and/or rules_go.
2. We document that defaults are "fallbacks", and may be overridden. Explicit gazelle_override will always override path-specific overrides and fallbacks.

Two changes that should be added in this PR (which I will add), are:

1. Remove anything from default_gazelle_overrides.bzl that isn't explicitly needed to build rules_go and gazelle.
2. Update documentation to include the overrides included in default_gazelle_overrides.bzl, the explicit extent of gazelle_default_attributes, and how users should use gazelle_overrides to override the small number deps in default_gazelle_overrides.bzl.

[fmeum]

I still think that the way you implemented precedence is the right one: Every module with Gazelle-provided defaults works if you don't specify a default override and if you do, you explicitly give up convenience for flexibility.

I think that's fine as long as we restore the list of defaults.

[fmeum]

The PR description is not up-to-date anymore, it still describes the previous precedence rule.

[tyler-french]

The PR description is not up-to-date anymore, it still describes the previous precedence rule.

Good point, updated.