Encode flag setting in InstalledPackageInfo? cabal doesn't rebuild installed packages when flags change.

**Describe the bug**
In https://github.com/haskell/text/issues/487 we noticed that there is no robust way to force `cabal-install` to reinstall a pre-existing library when flags are changed. In this case this means that there is no way for a user to enable the `text` package's `simdutf` flag if GHC ships with the flag disabled.

This is due to the fact that the only information `Cabal` has about pre-existing packages is that which is recorded in `InstalledPackageInfo`. As `InstalledPackageInfo` includes no knowledge of flags, `cabal-install` merely assumes that the pre-existing package's flag setting is compatible with that requested by the user.

For this reason, it seems reasonable to encode flag settings in `InstalledPackageInfo`. As, to my eyes, flags seem to be a slightly "higher-level" concern that the other `InstalledPackageInfo` fields (which encode precisely the information needed by GHC to compile and link against the package), I suspect this information best belongs in a "non-normative" IPI field. That is perhaps we introduce the convention that any field starting with `X-` will be ignored by GHC. This would allow `Cabal` to encode flag settings in an `X-Flag-Setting` field while keeping separate concerns separate and making it clear that GHC will not depend upon this knowledge.