avoid running expensive 'module use' command when using Lmod as modules tool, update $MODULEPATH directly instead

[Flamefire]

I discovered an issue: The priority.
There are 2 problems:

• module use foo won't put foo to the front if any other path has a (big) priority
• We have a function prepend_module_path with an optional priority. Hence if we don't pass a priority then this function will not "prepend" the path

@boegel

First bullet is not a problem, that's just how module use works?
Second one: yeah, if we really want to be sure, we should give it a (very) high priority, or at least detect that we need to do so.

It is a problem if I replace module use by setting MODULEPATH because that would yield a different behavior. I currently check if any priority is in use and fallback to module use instead
We use the priority in load_fake_module, but I guess we should not. The problem is: It doesn't tell what is intended. In prepend_module_path we could (hard, but possible) check current priorities and choose a higher one. Who knows that the 10000 we currently use isn't in use already or a user used 10001?
hence I'd deprecate the priority argument

[boegel]

lgtm

[Flamefire]

Follow up issue for the unadressed points: #3631
BTW: As you added tests for module unuse I think we should add the same handling for the unuse function: If no priority in effect, simply remove it.