This cleans up the code quite a bit, and also makes the next commit much
easier.

The intended use case is for docs.rs, which can now copy exactly the
files it cares about, rather than having to guess based on whether they
have a resource suffix or not. In particular, some files have a resource
suffix but cannot be shared between crates: rust-lang/docs.rs#1312 (comment)

The end goal is to fix rust-lang/docs.rs#1327
by reverting rust-lang/docs.rs#1324.

This obsoletes `--print=unversioned-files`, which I plan to remove as
soon as docs.rs stops using it.

This also changes custom themes from Toolchain to Crate files.

…d be invocation specific