Move path filesystems to a separate package.

Callers can apply the following gofmt rules:

gofmt -w -r 'fuse.NewPathNodeFs -> pathfs.NewPathNodeFs' $*
gofmt -w -r 'fuse.FileSystem -> pathfs.FileSystem' $*
gofmt -w -r 'fuse.LoopbackFileSystem -> pathfs.LoopbackFileSystem' $*
gofmt -w -r 'fuse.LockingFileSystem -> pathfs.LockingFileSystem' $*
gofmt -w -r 'fuse.PathNodeFs -> pathfs.PathNodeFs' $*
gofmt -w -r 'fuse.PathNodeFsOptions -> pathfs.PathNodeFsOptions' $*
gofmt -w -r 'fuse.DefaultFileSystem -> pathfs.DefaultFileSystem' $*
gofmt -w -r 'fuse.NewLoopbackFileSystem -> pathfs.NewLoopbackFileSystem' $*
gofmt -w -r 'fuse.NewLockingFileSystem -> pathfs.NewLockingFileSystem' $*
gofmt -w -r 'fuse.CopyFile -> pathfs.CopyFile' $*