Avoid installing eggs from wheels when possible

This relaxes commit ff2ee218
("Do never install from wheels") because for some eggs like plantuml,
there's no tarball.

About the implementation, no need to sort by version (as it was done)
because the list is already filtered to keep the best one.
2 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Buildout.UnitTest-Master.Python2

00:09:03

passed SlapOS.Buildout.UnitTest-Master.Python3

00:17:12