Resolves #2547

I introduce SimpleLicenseExpression to make "OrAnyLater LicenseRef"
unrepresentable. That also simplifies types.

license field is parsed as old `License` when cabal-version <2.2,
and as SPDX expression otherwise. `NONE` is recognised.

There are best-effort functions to convert between `License` and
`SPDX.License`.

There are also IPI changes: parser accepts both `License` and
`SPDX.License`, as both can occur in package database. Cabal will
`register` a SDPX expression as `license` for GHC >= 8.4, and legacy
`License` for other (we are smart when converting `PackageDescription` +
LBI and other data to `InstalledPackageInfo`)

Also add NFData InstalledPackageInfo