[pm] refactor and introduce fsnotify

This is preparation work for an ephemeral server based on the package output
manifests, which will introduce a new serve subcommand that shares much of
this code, but watches more files.

Test: tests are added at the end of the patch chain, due to being missing previously.
Change-Id: Ifebc55e5052392a8744c8d6291a4d59e806e855c