Incomplete module for extensions

[Flamefire]

I just noticed that the SciPy-bundle module generated by --module-only is missing some paths.

Those are supposed to be added by the numpy extension. However the current mechanism is flawed:

• The result of ext.run() is collected in a variable:

  easybuild-framework/easybuild/framework/easyblock.py

  Lines 1950 to 1952 in bf0af10

  	txt = ext.run()
  	if txt:
  	self.module_extra_extensions += txt

• This is then added to the module

  easybuild-framework/easybuild/framework/easyblock.py

  Line 1466 in bf0af10

  lines = [self.module_extra_extensions]

The problems with this are:

1. For --module-only this is not run
2. For --skip it might not be run
3. For --parallel-extension it is not used either:

   easybuild-framework/easybuild/framework/easyblock.py

   Line 2014 in bf0af10

   ext.postrun()

From our "official" easyblocks only numpy uses that mechanism.

So we need a new method for extensions that returns the module-extra when used as an extension. The current make_module_extra cannot be used for that.

I'd propose make_module_extra_extension but we already have make_module_extra_extensions which would be confusing.

[Flamefire]

@boegel I'd also consider this a blocker for 5.x especially as it is pretty much impossible to fix this in a fully backwards compatible manner, see the PR: #4652