ghc-session: Use new project building API
ghc-session: Use new project building API

One caveat: psiConfigure used to --only-configure the entire project. In
theory we shouldn't even need to do that anymore because we reconfigure
just the unit/target we need to before reading unit info.

However cabal has a bug or well they might consider it just inconsistent
behaviour in that instantiated backpack units' targets are not built by the
target mentioned in plan.json so this per-unit reconfigure is currently
broken there.

The workaround is to just build the entire project before running a query
for now.

DanielG/cabal-helper