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).
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