Relocatable build tree

[LaurentRDC]

This PR re-purposes the --enable-relocatable flag to make all paths in the build tree relative to the build tree root. This means that the build tree (e.g. dist-newtyle) can be copied wholesale elsewhere (for example, to a git worktree) and promptly be re-used by cabal without re-building.

This behavior can be triggered with cabal build --enable-relocatable, or the project-level configuration option relocatable: True.

Some notes:

• --enable-relocatable changes plan.json to relativize paths. It's not clear if that's too much of a breaking change.

• --enable-relocatable is not the default; it is opt-in.

• --enable-relocatable is backwards compatible, in a sense: older versions of cabal will see a stale build tree, and rebuild, instead of erroring out.

• --enable-relocatable is only needed for the initial build: the following does not trigger a rebuild:

  $ cabal build --enable-relocatable
  $ cp -R ./dist-newstyle ../some-worktree
  $ cd ../some-worktree
  $ cabal build
  < Up to date >

Fixes #12137

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