Add extension_name property to EasyBlock & do NOT inherit parent extension_name

[Flamefire]

As it is expected that the fallback of extension_name to name is often useful it makes sense to have a unique place for that.

The test discovered a critical flaw: We need to reset extension_name in Extension.__init__

@boegel Can you add this to the next release before we ship the extension_name feature?
Can also extract the bugfix although the test is easier with the property

Related: Shall we support templates in extension_name of extensions?
We currently support templates in the name (why? I don't see a use case) and I kept the existing mechanism in #5259 such that:
('foo', 1, {extension_name: "%(name)s-%(version)s"}) would resolve extension_name to foo-1 but I'm not sure that makes sense and it would complicate things for us if we want to use extension_name to derive template values as we would need to resolve extension templates once, then resolve extension_name, then resolve again, i.e. what we do in

easybuild-framework/easybuild/framework/easyblock.py

Lines 703 to 710 in 2a58220

	# construct dictionary with template values;
	# inherited from parent, except for name/version templates which are specific to this extension
	template_values = copy.deepcopy(self.cfg.template_values)
	template_values.update(template_constant_dict(ext_src))
	# Resolve this here where we have the template values available
	if 'extension_name' in ext_options:
	ext_options['extension_name'] = resolve_template(ext_options['extension_name'], template_values)

[boegel]

lgtm

[Flamefire]

@boegel Have you considered this question? Would need deciding before shipping.

Related: Shall we support templates in extension_name of extensions?