Add find_files builtin, use it to fix find_and_copy implementation

The macro find-and-copy finds all the files in the given source tree that
match the given filename patten and create <source>:<dest> pair with the
same relative path in the destination tree.

Bug: 193540681
Test: rbcrun build/make/tests/run.rbc
Change-Id: Ic4315ce2fab7a7791ab55dd9eed039205a1c721a