Refactor packages tags finder to use packages class

What does this MR do and why?

Considering the major refactoring related to the decomposition of packages table, the tags finder won't be able to rely on STI column anymore.

Given that, we're replacing package_type parameter with packages class in the Packages::TagFinder.

References

Foundation: Prepare application code to read fr... (#548761)

Screenshots or screen recordings

No.

How to set up and validate locally

1. Enable the feature flag Feature.enable(:packages_tags_finder_use_packages_class)

2. Publish npm package

   $ mkdir madrid && cd madrid
   $ npm init
   # Set package name according [the naming convention](https://docs.gitlab.com/user/packages/npm_registry/#naming-convention).
   # Setup [authentication](https://docs.gitlab.com/user/packages/npm_registry/#authenticate-to-the-package-registry)
   $ npm publish

3. Add a new tag to npm package docs

   $ npm dist-tag add @scope/package@version my-tag
   $ npm dist-tag ls @scope/package # verify that the tag was added

4. Remove a tag from npm package docs

   $ npm dist-tag rm @scope/package@version my-tag
   $ npm dist-tag ls @scope/package # verify that the tag was removed

MR acceptance checklist

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Related to #548761