Handle expressions in module use statements in ModulesTool.modpath_extensions_for

[geimer]

With #1472 in place, ModuleGenerator.use may generate module use statements including Tcl/Lua expressions that are evaluated at module-load time. The ModulesTool.modpath_extensions for method, however, only parses "regular" module use statements referring to a simple, quoted path. With the current usage model, this should be fine. However, if at some point in the future someone uses the prefix argument of the ModuleGenerator.use also for module paths that need to be considered in the modpath_extensions_for routine, things will break (for a potential use case, see below).

And now the potential use case: Assume that we have a system-wide software installation using EB in <prefix>, using a hierarchical module naming scheme. While it is already now possible to extend it with user-local software/module collection in $HOME (this is what #1472 was good for), it should in the long run also be possible for the user to use EB for installing this stuff (e.g., using --installpath=$HOME/.local). One example might be to install a special debugging version or a version with a specific patch of an MPI library, and then some software on top of it. Then, the module paths in the user's $HOME and the modpath extensions of modules installed there (which include such module use statements with expressions) need to be taken into account in the modpath_extensions_for.

Note: Presumably a bunch of other things need to be resolved before the use case above will work with EB.

[geimer]

Addendum: The conversation in #1472 also already discusses some caveats to consider when implementing this.