fix local search of manifests
fix local search of manifests

Since #1065 it was failing to find the unversioned `manifest`
files inside `<rocks_tree>/lib/luarocks/rocks-5.x/`. The result
was that `luarocks install foo` worked but then `luarocks show foo`
didn't.

luarocks/luarocks,keplerproject/luarocks,luarocks/luarocks,luarocks/luarocks,keplerproject/luarocks,keplerproject/luarocks,keplerproject/luarocks