Not all transitive references are corrected by require_explicit_imports

Describe the bug

We use require_explicit_imports: true, and have some found some cases where SwiftLint does not add imports for transitively used swift modules. The ones I've seen so far are:

1. Use of functions defined on extensions, where the module that defines the extension is not directly imported by the module that references the extension
2. Calling functions that have parameters with default values, and where the function/initializer of the default is defined in a module not directly imported
3. References to type metadata from modules not directly referenced. For example, one case was self.mapView.rx.willMove, and the module that defines the type of willMove was not being directly imported (it may also be relevant that the type of willMove is a generic and it conformed to a protocol)

In these cases, if the import does exist, the unused_import rule will remove it, which prevents manual fixes.

The main purpose of this issue is to see if these have been discussed, and to see if these implicit references are visible from SourceKit. These cases were identified when switching to dynamic linking, instead of static linking.

Start with the require_explicit_imports and unused_import rules, then investigate whether SourceKit exposes the transitive references described: extension functions, default-value functions or initializers, and type metadata. Done means documenting the findings and determining how implicit references should be handled without unused_import removing needed imports.