Search for mismatched package versions

[hololeap]

This came up because of gentoo-haskell/gentoo-haskell#1381. When process was upgraded, there may have been packages left on the system that are still built using the old process. This was because the package was never added to ebuilds that depend on it and therefore they were missing dev-haskell/process:= in RDEPEND (and therefore never rebuilt).

While this scenario should be rare, haskell-updater could be modified to detect situations like this. The basic process would look like:

1. Scan installed haskell packages and collect the package/version of each of their dependencies. This could be through scanning package.conf.d or using a higher-level interface such as ghc-pkg.
2. When all of the dependencies are collected, look for any that are listed with more than one version, then determine which one is preferred (highest version number). Flag the other versions as non-preferred.
3. Find any packages that are still depending on the non-preferred version and then pass them off for a rebuild.