MDLSITE-2720 Enforce "plugin" for 2.7 and up

Before 2.7, it was allowed to use both "module" and "plugin"
in module activities. For 2.7 and up we only accept "plugin"
in core. Note that for non-core module activities "module" will
be allowed (deprecated) up to 2.10. No matter of that, we report
any "module" use as error here.