See discussion on #1920

From @tgamblin

Yeah, sorry for that. That is a concretization issue. The concretizer thinks that if a package has a single version that is not a range or a list, that it's concrete, so if you say 2.7 on the command line it never gets matched against known versions. But 2.7, if compared to other versions, will be satisfied by 2.7.1, 2.7.8.1, etc.

Probably a version needs to know whether it is exact, so the concretizer doesn't have to use a kludgy test like it does. This would also be useful for users -- I have thought about allowing "exact" version literals like 2.7. (period at the end) to indicate the version is exactly v2.7 and shouldn't match anything else.