This moves `git.FileSet` to `libs/fileset` and decouples it from the Git package.
It is made aware of gitignore rules in parent directories up to the
repository root as well as gitignore files in underlying directories
through the `fileset.Ignorer` interface.
The recursive directory walker is reimplemented with [filepath.WalkDir].
Follow up to #182.
Pieter Noordhuis