bzl_library does not validate the inputs are correct

[alexeagle]

As a common example, many rulesets do not declare all the deps of their .bzl files.

Ever since Bazel 7 added the native https://bazel.build/reference/be/general#starlark_doc_extract rule, it makes sense for that to be the behavior of a bzl_library target.

This is symmetric with proto_library which produces a descriptor as an output.

There's a design smell where a macro has to do this:
https://github.com/bazelbuild/stardoc/blob/master/stardoc/stardoc.bzl#L112-L119
which means that alternative tools that want to read the docs don't know which target to refer to in order to gather that module-as-proto data.

Ideally what should happen:

• starlark_doc_extract in Bazel should have a java_binary that runs the tool outside of that native rule implementation (starlarkification)
• we can attach a validation action to bzl_library targets that executes that tool
• as a bonus, we can also add an OutputGroup to provide that proto data as output

[alexeagle]

Note, I have forked the bzl_library rule to aspect_bazel_lib because it doesn't seem like #571 is likely to be accepted.