Also use the new ghcup for bootstrapping GHC in install_deps.

This should be backwardscompatible with 0.0.1 json format.

Also slightly change 'getTagged' to list the latest version
with a tag, not the oldest.

Fixes #11

Fixes #7

Wrt #7

Fixes #2

This should be the *parent* dir of '.ghcup', not
the full destination.