Historically the package db format was a single text file in Read/Show
format containing [InstalledPackageInfo]. For several years now the
default format has been a directory with one file per package, plus a
binary cache.

The old format cannot be supported under the new scheme where the
compiler will not depend on the Cabal library (because it will not
have access to the InstalledPackageInfo type) so we must drop support.
It would still technically be possible to support a single text file
style db (but containing a different type), but there does not seem to
be any compelling reason to do so.

(Part of preparitory work for removing the compiler's dep on Cabal)