push tasks :file and :pom options don't search in fileset

The code to find the jar and pom file given to a `push` tasks uses `io/file` which essentially prevents you from specifying a file in the fileset. This in turn requires you to output things into `target` to make things work.

Proposal: We search for both files in the following order:

- exact fileset match
- classpath match (io/resource... )
- file match (io/file ...)

Also exceptions around some of this stuff could be improved. If there's a realistic chance to get this into 2.7.2 I'd be very happy to start working on this ASAP.

Pinging @Deraen @arichiardi @SevereOverfl0w for opinions :)

Trace the push tasks' :file and :pom handling, starting with the current io/file lookup and its interaction with filesets. Compare the relevant io/resource behavior, then verify that lookup follows exact fileset, classpath, and file matches in order and that the related exceptions are clearer.

Written by the indexing model from the issue text.