Hadrian: Add support for building stage3

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 (closed)
2. haddock and ghc-tags are built as stage1 executables (with the stage1 compiler) rather than as stage2 compiler. Fixing hadrian#661

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