spack v1.0 support: %foo +bar -> +bar %foo

[haampie]

Spack v1.0 will parse pkg %gcc +foo as "pkg with direct dependency gcc
with variant foo enabled" instead of "pkg with variant foo enabled and compiler
gcc". Reorder as pkg +foo %gcc to be compatible with Spack v0.x and
v1.0.

Summary

• This PR is a (refactoring, bugfix, feature, something else)
• It does the following (modify list as needed):
  • Modifies/refactors (class or method) (how?)
  • Fixes (issue number(s))
  • Adds (specific feature) at the request of (project or person)

Design review (for API changes or additions---delete if unneeded)

On (date), we reviewed this PR. We discussed the design ideas:

1. First idea or goal
2. Second idea
3. Third idea

This PR implements 1. and 3. It leaves out 2. for the following reasons

• (impractical)
• (too big)
• (not a good idea anyway)

[kennyweiss]

Thanks @haampie !