Limit binding generation to certain headers

After #974, we should be apply to apply a filename filter to the items generated e.g. just generate items in "foo/bar".

We should also be able to specify a Rust package root to learn about types relating to other packages.

This will enable multiple `include_cpp!` macros to work together.

Start by reading the changes in #974 and the existing include_cpp! behavior. Determine how filename filtering and a Rust package root should interact, then verify that multiple include_cpp! macros can work together without generating unrelated items.

Written by the indexing model from the issue text.