ENH: Add custompre: variant of custom: that executes before native in…

…stall

This needs to be mostly duplicated (data structures, functions), even though the code is identical. Might employ metaprogramming (like elsewhere) to unify this more.
Use case is triggering a reinstallation (of nodejs) after a different APT repo is chosen. (But wait, we have an apt-reinstall: for that!)