Multiple global package databases

Currently, we hardcode the global package database path to somewhere in the Cabal datadir of the asterius package, and ahc-boot populates the package database by executing the booting process. This is a major source of coffee break sessions in the past: whenever we need to modify something in the boot libs, we'll need to wipe the ghc-toolkit/asterius installation, rebuild and do another round of ahc-boot.

We should have a default global package database path which falls back to the Cabal datadir, but additionally:

• Allow the source of boot libs to be picked up from the current working source tree, instead of from the datadir of ghc-toolkit (which requires recompilation of ghc-toolkit and asterius)
• Allow ahc-boot to use a new global package database path
• Allow ahc-boot to only compile a part of boot libs, e.g. "up to package X", and later resume the compilation of the rest of boot libs
• Allow ahc, ahc-link, etc to use custom global package paths

In this way, the experience of hacking boot libs can be much smoother. Still, we're free to schedule coffee breaks whenever we want.