This supports the feature of newer ghc-pkg version which have the
register --enable-multi-instance flag. This allows registering multiple
instances of the same version of a package into a single database.

In addition, to support the same feature on some older ghc-pkg versions,
the HcPkgInfo has a recacheMultiInstance capability, which tells us if
the trick of registering multiple instances by running ghc-pkg recache
works. This is known to work for all versions of ghc-pkg that support
the recache command at all.

Then HcPkg.registerMultiInstance will use one of the two methods
depending on which is supported, or fail if neither is. Currently only
registering into specific package dbs is supported, not global or user.

This new multi-instance feature is needed for cabal-install.