specified easyblock for extension is not taken into account

[arielzn]

If we specify e.g. in a PythonBundle easyconfig , to install an extension that has the same name of an existing EasyBlock , easybuild will try to use that easyblock for it.

A check should be added to honor an easyblock: definition on the extension dict as e.g. :

    ('tbb', '2021.2.0', {
        'easyblock': 'PythonPackage',
        'source_tmpl': 'tbb-%(version)s-py2.py3-none-manylinux1_x86_64.whl',
        'unpack_sources': False,
        'checksums': ['bf54992d1b5d6135620d5f3f2649bf331e2c412a4af00effd35481a9ecb98ee6'],
    }),