This defines the new cabal.project files and introduces the notion of a
project root (and the logic for finding it). Also has support for
implicit projects when no cabal.project file is defined.

Supports both reading and writing project files or fragments. The
printing & parsing round trips correctly. QC tests to follow.

This is a key part of the new nix-local-build branch approach, based
around projects with clear configuration state held in a project file
(or extra files).

This has support for file and dirs as packages within a project,
including by glob. It supports both globs that much match a target, and
optional globs that are allowed to match nothing. It has partial support
for local tarball, remote http tarball and remote source repo packages.