This ticket enables the building of a `stage3` compiler by making the
build logic more consistent and predictable in Hadrian.

Two of the main changes are:

1. In order to build anything at stageN we use the package database
present at stageN. Fixing #16069
2. `haddock` and `ghc-tags` are built
as stage1 executables (with the stage1 compiler) rather than as
stage2 compiler. Fixing
[hadrian#661](https://github.com/snowleopard/hadrian/issues/661)

In order to build a stage3 compiler, you have to set the new `finalStage` hadrian option to `Stage3`.