[clang][deps] NFC: Split out the module-based API from the TU-based API

For users of the C++ API, the return type of `getFullDependencies` doesn't make sense when asking for dependencies of a module. In the returned `FullDependenciesResult` instance, only `DiscoveredModules` is useful (the graph of modular dependecies). The `FullDeps` member is trying to describe a translation unit it was never given. Its command line also refers to a file in the in-memory VFS we create in the scanner, leaking the implementation detail.

This patch splits the API and improves layering and naming of the return types.

Depends on D140175.

Reviewed By: artemcm

Differential Revision: https://reviews.llvm.org/D140176