New module for new style project configuration files

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.

(cherry picked from commit 324b324)

cabal-install/Distribution/Client/Config.hs

@@ -39,7 +39,9 @@ module Distribution.Client.Config (
     withProgramOptionsFields,
     userConfigDiff,
     userConfigUpdate,
-    createDefaultConfigFile
+    createDefaultConfigFile,
+
+    remoteRepoFields
   ) where
 
 import Distribution.Client.Types

cabal-install/Distribution/Client/Dependency/Types.hs

@@ -261,6 +261,9 @@ data ConstraintSource =
   -- | Main config file, which is ~/.cabal/config by default.
   ConstraintSourceMainConfig FilePath
 
+  -- | Local cabal.project file
+  | ConstraintSourceProjectConfig FilePath
+
   -- | Sandbox config file, which is ./cabal.sandbox.config by default.
   | ConstraintSourceSandboxConfig FilePath
 
@@ -298,6 +301,8 @@ instance Binary ConstraintSource
 showConstraintSource :: ConstraintSource -> String
 showConstraintSource (ConstraintSourceMainConfig path) =
     "main config " ++ path
+showConstraintSource (ConstraintSourceProjectConfig path) =
+    "project config " ++ path
 showConstraintSource (ConstraintSourceSandboxConfig path) =
     "sandbox config " ++ path
 showConstraintSource (ConstraintSourceUserConfig path)= "user config " ++ path