misc out of tree/side install fixes

Two fixes found testing with side-installed flux-core and out of tree component builds

1) Lua environment was not set up right by flux command driver if flux-core is built with --prefix that does not map to default lua search paths.  Make sure the installed paths end up in the environment.

2) The broker and flux-module algorithm for determining whether a module is specified by path or by name was stupid.  It would find 'barrier' (a command in the cwd) before barrier.so on its search path.  Now we require there to be a "/" in the name for a module name to be treated as a path.