[clang][deps] Remove `ModuleDeps::ImportedByMainFile`

This information is already exposed via `TranslationUnitDeps::ClangModuleDeps` on the `DependencyScanningTool` level, and this patch also adds it on the `DependencyScanningWorker` level via `DependencyConsumer::handleDirectModuleDependency()`.

Besides being redundant, this bit of information is misleading for clients that share single `ModuleDeps` instance between multiple TUs (by using the `AlreadySeen` set). The module can be imported directly in some TUs but transitively in others.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D156563