The config option for choosing either `modulecmd` or `lmod` will be added in an upcoming PR.
I want the PRs to be manageable; this one only cleans up what was contributed in #616, and doesn't add anything new on top of that.

Following this PR, there will be PRs for a configure option to pick either `lmod` or `modulecmd`, to add support for `modulecmd.tcl` (Tcl environment modules as opposed to the C implementation), creating Lua module files, etc.