Version ranges and geq condition for any_version in rules files

[NikolausDemmel]

Here is a proposal for how to realize this:
http://xylem.readthedocs.org/en/latest/spec.html#any-version-and-version-ranges

This does require os plugins to provide an order on versions for rule lookup, but not for rule file loading / expanding / saving.

Feel free to comment / discuss in this issue.

[NikolausDemmel]

cc @wjwwood @tfoote @dirk-thomas @esteve

[tfoote]

Looks reasonable. I'd also clarify on a collision who wins in a case like this:

precise, raring, utopic: [foo]
any_version>=trust: [bar]

Presumably the explicit rule would take precedence.

[NikolausDemmel]

Yes, the explicit rule you win. I do not want to forbid this, since that would again make rule file parsing dependent on the OS plugin knowledge.

One could argue that the test suite for rosdistro could check and forbid this, but I don't see a real need. Also the following seems reasonable to me:

foo:
  osx:
    any_version>=mavericks:
      homebrew: libfoo
    mavericks:
      macports: libfoo-1
    yosemite:
      macports: libfoo-2