exts_classmap should be removed, and replaced by an ext_option option

[JensTimmerman]

Classmap is another way of telling the framework something, but we already have ext_option to convey information on a per extension base, so we should use this instead.

Also, classmap is a misnomer, since, what are classes in an EB context?

[boegel]

should be in the easyblocks repo, please move it

[boegel]

This is a bit of a misnomer, but we still need it for now (even though it's not actually used in the central easyconfigs repository), since it's a fallback for exts_defaultclass (also a misnomer I guess) and trying to use an extension-specific easyblock.

We'll considering deprecating it in EasyBuild 4.x...