Rollup merge of rust-lang#147923 - celinval:rpub-remove-trait, r=oli-obk

Simplify rustc_public context handling

We no longer need two thread-local variables to store the context. We used to have two because the conversion logic used to live in a separate crate from the rest of the business logic.

I'm also merging the Container struct and the CompilerInterface trait as a single struct. This removes the unnecessary indirection and code duplication. Using a trait would also block us from adding any generic method to the compiler interface.

r? `@oli-obk`

cc: `@makai410`

Author: Zalathar