Sylvain Henry authored Jan 15, 2020 and Marge Bot committed Jan 31, 2020

The package terminology is a bit of a mess. Cabal packages contain
components. Instances of these components when built with some
flags/options/dependencies are called units. Units are registered into
package databases and their metadata are called PackageConfig.

GHC only knows about package databases containing units. It is a sad
mismatch not fixed by this patch (we would have to rename parameters
such as `package-id <unit-id>` which would affect users).

This patch however fixes the following internal names:

- Renames PackageConfig into UnitInfo.
- Rename systemPackageConfig into globalPackageDatabase[Path]
- Rename PkgConfXX into PkgDbXX
- Rename pkgIdMap into unitIdMap
- Rename ModuleToPkgDbAll into ModuleNameProvidersMap
- Rename lookupPackage into lookupUnit
- Add comments on DynFlags package related fields

It also introduces a new `PackageDatabase` datatype instead of
explicitly passing the following tuple: `(FilePath,[PackageConfig])`.

The `pkgDatabase` field in `DynFlags` now contains the unit info for
each unit of each package database exactly as they have been read from
disk. Previously the command-line flag `-distrust-all-packages` would
modify these unit info. Now this flag only affects the "dynamic"
consolidated package state found in `pkgState` field. It makes sense
because `initPackages` could be called first with this
`distrust-all-packages` flag set and then again (using ghc-api) without
and it should work (package databases are not read again from disk when
`initPackages` is called the second time).

Bump haddock submodule