Feature request: Allow module version selection

[finnerty-fzj]

Tcl environment modules use a .version file to specify the default version to load amongst a set of versions. It would be good to be able to be able to generate such files as this would make this tool more usable for shared deployments.
The modules file tree is something like.

- ${PACKAGE}/
     - .version
     - ${VERSION}/
         - .version
         - ${VARIANT}

Where the .version file contains the version to use when users load a tool without giving a version number. The following example makes 3.3 the default NEST (assuming only one module file or a .version file in the ${VERSION} sub-folder).

#%Module1.0
## Default NEST version
set ModulesVersion 3.3

Perhaps have shell variables in the plan files, like the following, that create the version files if they are set otherwise do nothing. This would require some decisions about what to do if multiple plan files have these set.

MODULES_DEFAULT_VERSION=...
MODULES_DEFAULT_VARIANT=...

Alternatively these could be created in an optional separate step like

    build.sh set_module_default_versions [package] [version] [variant:-default]