Remove short-name hacking

The problem

In e.g. src/fgen/templates/calculator_manager.f90.jinja, src/fgen/templates/wrap_module.f90.jinja and in line 100 or so of src/fgen/templator.py there is this hard-coded "mod+" that goes in front of the module's short-name. This is super confusing when you're trying to link stuff, because you have no control over what this prefix is or the fact that it will appear. We should fix this so it's clear to users what name they need to use when linking things.

Definition of "done"

  • the module prefix is used consistently throughout and taken from some common configuration place (rather than being hard-coded in some places)
  • document how this module prefix is handled so people can write the linking information without massive headaches

OR

  • remove the idea of short-names and prefixes entirely, it's not clear to me that they're that helpful

Additional context

Came out of !25 (closed). I was trying to wrap a module that didn't start with mod_ and ended up being extremely confused about how to get it to link properly until I found the hard-coded mod_ floating around.