This allows users to get a JSON representation of various information
about how Cabal would go about building a package. The output of this
command is intended for external tools and therefore the format should
remain stable.

Add (currently nonfunctional) new-show-build-info
Fix compile error
Make new-show-build-info functional
Use silent verbosity by default on showBuildInfo commands to keep output json clean
Make show-build-info commands hidden
Implement write-autogen-files
Make new-write-autogen-files work
Make new-write-autogen-files configure if necessary
Use target selectors for new-show-build-info
Don't prune plan for new-show-build-info
Only configure in new-show-build-info and new-write-autogen-files if no persist build info file is found
Wrap multiple target output of new-show-build-info in json list

Also, rename new-show-build-info to show-build-info in ShowBuildInfo

This means that cabal-install now extracts the LocalBuildInfo etc.
itself for each component, and now assembles the JSON without the need
for writing to temporary files. It also means that one build info JSON
object can be returned instead of an array. It works by configuring each
component separately as before, and instead of making its own build info
object, it just collects the component information.

This one build info object now reports the compiler used with the
ElaboratedSharedConfig, which is shared across all components.

Every other command defaults to what they used to do. show-build-info
now just chooses the first choice, since it doesn't care about
ambiguity.

These are needed by tooling to setup GHC sessions.

This fixes a lot of edge cases for example where the package db wasn't
created at the time of configuring. Manually doing the setup.hs wrapper
stuff was hairy.

It also changes the internal representation of JSON to Text rather than
String, and introduces the buildinfo-components-only flag in the Cabal
part to make it easier to stitch back the JSON into an array in
cabal-install.

Turns out we do need to keep the show-build-info part inside Cabal as we
rely on LocalBuildInfo which can change between versions, and we would
need to do this anyway if we wanted to utilise the
ProjectPlanning/Building infrastructure.