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 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`.