Add ModuleImplicitImportsRequest, which computes
the modules that should be implicitly imported by
each file of a given module. Use this request in
import resolution to add all the necessary
implicit imports.
The request computes the implicit imports by
consulting the ImplicitImportInfo, which ModuleDecl
can now be created with. This allows us to remove
uses of `SourceFile::addImports` in favor of
adding modules needed to be implicitly imported to
the ImplicitImportInfo.
