serve: Make the server find assets generated by external tools. When a processor is bypassing PieCrust's pipeline, we don't know what output files they have produced, so if we don't find an asset in the record for a given path, we quickly check the output folder for any matching file there. If there is, just return it.

*.pyc
venv/
build/lib
build/messages/_cache
dist/
piecrust.egg-info/
piecrust/__version__.py