rename modextraalias to modaliases

@pescobar: minor tweak to https://github.com/hpcugent/easybuild-framework/pull/952, I feel the `extra` doesn't make much sense (since EB won't define any aliases itself), and it should be plural hence `aliases`

enhanced unit tests are fine, nice job

please merge this in ASAP so we can include this in EasyBuild v1.14.0