Add cabal.project to allow "cabal new-build"

Note: this is needed if any other project (such as HaRe) wants to use
"cabal new-build" having ghc-mod as a dependency.

Otherwise it is innocuous

.gitignore

@@ -19,3 +19,4 @@ cabal.sandbox.config
 cabal-dev
 /TAGS
 /tags
+/cabal.project.local

cabal.project

@@ -0,0 +1,3 @@
+-- do not build sub-packages when using features from
+-- http://blog.ezyang.com/2016/05/announcing-cabal-new-build-nix-style-local-builds/
+packages: .

ghc-mod.cabal

@@ -31,6 +31,7 @@ Data-Files:             elisp/Makefile
                         elisp/*.el
 Data-Files:             LICENSE COPYING.BSD3 COPYING.AGPL3
 Extra-Source-Files:     ChangeLog
+                        cabal.project
                         SetupCompat.hs
                         NotCPP/*.hs
                         NotCPP/COPYING