add support for using custom module file template (WIP)

[boegel]

Adds support for specifying a custom module file template via --modules-template, as well as for dumping the default modules template:

$ eb --show-default-modules-template
$shebang$header$descr$group_check$deps$modpath$env_vars$footer

This should meet the use case of @ax3l discussed in #2624, since it allows using a custom template like:

$shebang$header$descr$group_check$deps$modpath$env_vars$footer
prepend_path("CMAKE_PREFIX_PATH", "%(installdir)s")

In some sense this is already supported via --modules-footer though, so the added benefit is limited for that particular use case, but it does provide more control over the generated module files...

(WIP since tests are missing)

[ax3l]

Thank you for the great feedback and proposed update!
I can imagine several cases where one wants to add vars such as MANPATH, additional program-specific helper vars, etc. Maybe one can parametrize the module template even on the easyblock?

As a background and comparison to spack, one could export/extend vars on a per-package basis and a general filter (all, all depending on X, etc.)

[boegel]

@ax3l So, rather than being able to provide a single fixed module file template that is used for everything, you'd like this be (potentially) be conditional, such that a different template is used in different situations?

That's way beyond what I had in mind for this... I guess this can be a first step.

This could be extended later to support that the --modules-template value points to a Python module/script that generates a template based on the availabe inputs... Is that what you're looking for?

[akesandgren]

Considering that you can do basically anything you want with the easyhooks, is this still relevant?