repo: repository directive

[blico]

Implements #132

A repository directive specifies a repository rule that Gazelle should know about. The directive can be repeated multiple times and can be declared from within a macro definition that Gazelle knows about. At the very least the directive must define a rule kind and a name attribute, but it can define extra attributes after that

[jayconrod]

Thanks for working on this. Generally looks good with a few comments below.