Enforce correct variant usage for rust_bindgen.

Modules defined in the srcs property are automatically added as
dependencies with AddDependency(), which will use any variant available.
This can cause incorrect architecture bindings to be silently pulled in,
such as when a host module uses a rust_bindgen module that doesn't
create a host variant.

This moves populating depPaths.SrcDeps over to depsToPaths and adds a
check for SourceProviders to make sure the correct OS and architecture
is being used.

Bug: 161826371
Test: Soong no longer silently pulls in bindings for the wrong
      target.
Test: New Soong test to catch this case passes.
Change-Id: I2b3651cf6fc7dabf4081434df1c455e637f5b3a4