Merge&sort imports from the same package

[mrkam2]

Prefer this canonical form:

  (:import
    (com.google.protobuf CodedInputStream CodedOutputStream))

over this repetitive:

  (:import
    (com.google.protobuf CodedInputStream)
    (com.google.protobuf CodedOutputStream))

Also sort class names within it, i. e. change

  (:import
    (com.google.protobuf CodedOutputStream CodedInputStream)

to

  (:import
    (com.google.protobuf CodedInputStream CodedOutputStream)

[cespare]

I intentionally left this out when implementing the "How to ns" rules because there are some hard-to-resolve questions about how, exactly, it should work.

To repeat what I wrote over there, what we need first is a set of rules/heuristics that yield good results, are easy enough to describe, and aren't too hard to implement.