use new golang.org/x/... subrepo naming (see https://docs.google.com/document/d/1VdB_ecg6wTBC-iRzAT-eCwvzj0rlRw7iL5KRdzGtlu4/edit)
update import paths: use sourcegraph.com/sourcegraph/... and golang.org/x/...
add method for accessing a walkable fs's underlying fs
add Walkable helper func to expose walkableFileSystem
automatically create a subFS's prefix dirs if they don't exist
remove Underlying (no longer necessary)
make WalkableFileSystem only require a readable VFS, not a read-write VFS
add ReadOnly
Revert "make WalkableFileSystem only require a readable VFS, not a read-write VFS"

This reverts commit f78b76f81a9f2bfc63c612a6ea254d187a86df48.
Merge branch 'permissions'