Avoid circular imports

[kraenhansen]

What, How & Why?

This closes #6565 by refactoring imports away from the "internal" pattern (which we introduced when we used Rollup to bundle into a single file) and introducing an indirect object which gets classes injected once they're initialized. This also untangles the recursively referencing collections accessors introduced with collections in mixed, into separate files.

This also adds a dependency in the "madge" tool enabling a new script to check for circular imports:

npm run check-circular-imports --workspace realm

Although this still fails with three cases of circular references 🤔 I need to decide how to deal with those before adding it as a command which is ran as part of our lint workflow on CI.

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 📝 Update COMPATIBILITY.md
• 🚦 Tests
• 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• 📱 Check the React Native/other sample apps work if necessary
• 💥 Breaking label has been applied or is not necessary
• 🔔 Mention @realm/devdocs if documentation changes are needed

[kraenhansen]

@gagik I recognize these are a lot of changes, but most were pretty mechanical. I'd love to get this in soon-ish to avoid the codebase from diverging while I'm PTO 🙏

[gagik]

Nice changes!

[kneth]

The imports are much easier to read. And I like that you have isolated the collection accessors in separate files.