Incorrect runtime.json processing

[analogrelay]

When restoring a package that has a runtime.json, we end up picking the least specific runtime ID, not the most specific runtime ID. As far as we can tell, this hasn’t produced any known critical issues, but it means we’re picking the wrong dependencies. For example, System.Net.Primitives has the following runtime.json:

{
  "runtimes": {
    "win7": {
      "System.Net.Primitives": {
        "runtime.win7.System.Net.Primitives": "4.0.11-beta-23401"
      }
    },
    "osx.10.10": {
      "System.Net.Primitives": {
        "runtime.osx.10.10.System.Net.Primitives": "4.0.11-beta-23401"
      }
    },
    "unix": {
      "System.Net.Primitives": {
        "runtime.unix.System.Net.Primitives": "4.0.11-beta-23401"
      }
    }
  }
}

On osx.10.10-x64, we end up picking runtime.unix.System.Net.Primitives.

/cc @Eilon @muratg @davidfowl @DamianEdwards @danroth27 @BrennanConroy - May be a candidate for beta8. I'll send an email shortly ;)

[muratg]

I think @BrennanConroy noticed this today, if I understand it correctly.

[analogrelay]

Correct, he pinged me, which started the investigation :)