Adds :mappings and :models modules

[JayShortway]

As the title says. Since we don't want to generate Dokka docs for internal libraries, I made some changes to minimize duplication.

• Moved all logic from LibraryConventionPlugin to separate functions.
• Renamed LibraryConventionPlugin to PublicLibraryConventionPlugin.
• Added a new InternalLibraryConventionPlugin.
• Created the new :mappings module, and applied the new InternalLibraryConventionPlugin.
• Set up the dependencies (:mappings depends on PHC. :core and :revenuecatui depend on :mappings.)
• Corrected the CocoaPods config where applicable, setting the correct iOS deployment target and version.
• Also added the new :models module, and applied the PublicLibraryConventionPlugin.
• Generated Mermaid graphs to visualize dependencies.

[JayShortway]

Drafted because we need to add :models.

[JayShortway]

@tonidero @vegaro I just added 2 commits:

• https://github.com/RevenueCat/purchases-kmp/pull/157/commits/421530595eae7fe16b17e40d412ddaf1ea605bf6 The first one adds the :models module.
• https://github.com/RevenueCat/purchases-kmp/pull/157/commits/f776e243c5511bcda1f617a42337b4adcb5ed95c The second one adds mermaid graphs to each module, generated with a Gradle plugin. (For now we need to manually run ./gradlew createModuleGraph to regenerate.) It's similar to what Now in Android does, e.g. here. They are automatically rendered on GitHub (e.g. see the :core module). IntelliJ also claims support, but haven't seen that working yet. Let me know if you think this is useful.