Be stricter when autodetecting `source.dir`
Be stricter when autodetecting `source.dir`

Only assume directory is the location when the tarball contains
only a single directory at the top level.

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