request: look up runfile matches given only filename

For our use case it would be really helpful if the runfiles library had an API that took a filename and returned a vector of matches for that filename. Clearly this will iterate the entire std::map and be quite expensive and for our use case that is fine. We are replacing lookup of an executable by name in our legacy build with lookup of the same thing via runfiles and having to update all calling sites with bazel path is challenging for us. Plus the name based search would allow us to move executables around in our repo without modifying the call sites.

Thanks! Chris

Start by locating the runfiles library's existing lookup API and the std::map that stores its entries. Add a filename-based lookup that returns all matching paths, then verify that it supports replacing executable-name lookups without requiring callers to use bazel paths.

Written by the indexing model from the issue text.