Generator 'pkg_config': same logic as others to get the names

[jgsogo]

Changelog: Fix: Generator pkg_config uses the same logic as the rest of them to get the name of the generated files.
Docs: omit

This PR rollback changes introduced in #6223 that were fixing some issues reported before cpp_info.names was introduced. Now that the recipe writer has full control over the generated .pc filename using cpp_info.names['pkg_config'] = 'xxxx' IMHO it makes no sense to keep this behavior here.

[jgsogo]

@memsharded @danimtb I really think we can (and should) get rid of this custom logic for this generator, wdyt?

[memsharded]

@memsharded @danimtb I really think we can (and should) get rid of this custom logic for this generator, wdyt?

I would really like to fix it, but as I see it, this would be breaking behavior. And most likely it will break pkg_config generator users.

[jgsogo]

Ok. let's make this change in Conan v2 and meanwhile I'll try a different approach