Replace MatchSpec with no name (`*`) with glob matchspec

[pavelzw]

i'm not sure whether we should make * a special case matchspec (i.e. name = None). how about we just make it a glob matchspec with * as the glob matcher? then we can probably simplify some things where we check for this.

wdyt @baszalmstra?

Originally posted by @pavelzw in #2036 (comment)