copy load_source function from easybuild.tools.py2vs3.py3 to easybuild.tools.filetools so it can be used in easybuild.tools.hooks

[boegel]

This fixes a problem that was introduced by syncing the 5.0.x with develop after #4280 got merged:

ERROR: test_get_easyblock_class_name (test.framework.filetools.FileToolsTest)
Test for get_easyblock_class_name function.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/runner/ec67a2f31722d0b0d37dc7b0811a2a5b4f48dc22/lib/python3.6/site-packages/test/framework/filetools.py", line 3020, in test_get_easyblock_class_name
    self.assertEqual(ft.get_easyblock_class_name(configuremake), 'ConfigureMake')
  File "/tmp/runner/ec67a2f31722d0b0d37dc7b0811a2a5b4f48dc22/lib/python3.6/site-packages/easybuild/tools/filetools.py", line 2782, in get_easyblock_class_name
    mod = load_source(fn, path)
NameError: name 'load_source' is not defined

We still need the load_source function since it allows avoiding to rely on the (deprecated) imp module from the Python stdlib, and since it's also used in easybuild.tools.filetools that's a good place to put it.