Extend HcPkg.init support for dir-style package dbs

The HcPkgInfo useSingleFileDb is split into two: supportsDirDbs and
requiresDirDbs. Then rather than HcPkg.init callers having to do the
writeFile [] thing, HcPkg.init does it itself automatically based on the
HcPkgInfo. In the case that supportsDirDbs is True but requiresDirDbs is
False then we have a choice, to use dir style or file style. For
compatability reasons, when using ghc/ghc-pkg for the inplace package db
we want to use the old file style, even though dir style is supported.
However in other circumstances (e.g. in places in cabal-install) we
would like to use the dir style if it's supported, and there are no
backwards compat issues. So HcPkg.init gains a new Bool arg to request
using the file style if it's still supported. Only this mode is used
within Cabal itself, but the non-compat mode is available for other
users.

The compiler-independent initPackageDB is left with the same old
behaviour, but a new createPackageDB has the extra compat argument
(which is only passed to hc-pkg for ghc-pkg).