Rule: missing-metadata-annotation

[anderseknert]

It should be possible to say that all rules of function matching a pattern, like "does not start with _", should have a metadata annotation.

[anderseknert]

While some enforcement of this rule could be done in the scope of a single file, this one too depends on #282 to be truly useful. Requirements like "any package must have a metadata annotation" can only be verified by looking at all packages in the project and not individual files, as a single package could be distributed across any number of files.