install.hs: Make all available GHCs in PATH buildable

[maoe]

This PR extends the GHC search logic so that install.hs can find all available ghcs in $PATH.

The motivation is that at our company we have our own build of GHCs installed in some directories with custom versioning (e.g. ghc-8.6.4.0) and make them available by modifying $PATH variable. Currently stack install.hs cabal-ghcs cannot find them because it only tries to find GHCs whose version matches one of the versions returned by getHieVersions.

The new search logic does the following:

• Find all known GHCs in $PATH
  • This is the original behavior.
• Find all available ghcs in $PATH
• Merge the two sets of paths.
  • If stack-provided GHCs are included, exclude them.
  • If both sets have the same version, the known GHC takes precedence.

This can also be useful to try hie with GHC alpha/HEAD etc. You can make the GHC available in $PATH and type stack install.hs cabal-ghc-8.7.20190618 for example.

[fendor]

waiting for approval of @power-fungus