Refactor the way packages databases are handled

- cradle now stores a list of active package databases instead of only
  the user store (if present).

- rename `cradlePackageDb` -> `cradlePkgDbStack` as that`s what the ghc
  documentaion calls this kind of thing

- `getPackageDbPackages` now returns names of all visible packages in
  the given directory. Also the implementation now uses `ghc-pkg`
  instead of manually looking at the package database

test/data/cabal.sandbox.config.in

@@ -4,15 +4,15 @@
 -- if you want to change the default settings for this sandbox.
 
 
-local-repo: /home/me/work/ghc-mod/test/data/.cabal-sandbox/packages
-logs-dir: /home/me/work/ghc-mod/test/data/.cabal-sandbox/logs
-world-file: /home/me/work/ghc-mod/test/data/.cabal-sandbox/world
+local-repo: @CWD@/test/data/.cabal-sandbox/packages
+logs-dir: @CWD@/test/data/.cabal-sandbox/logs
+world-file: @CWD@/test/data/.cabal-sandbox/world
 user-install: False
-package-db: /home/me/work/ghc-mod/test/data/.cabal-sandbox/i386-osx-ghc-7.6.3-packages.conf.d
-build-summary: /home/me/work/ghc-mod/test/data/.cabal-sandbox/logs/build.log
+package-db: @CWD@/test/data/.cabal-sandbox/i386-osx-ghc-7.6.3-packages.conf.d
+build-summary: @CWD@/test/data/.cabal-sandbox/logs/build.log
 
-install-dirs 
-  prefix: /home/me/work/ghc-mod/test/data/.cabal-sandbox
+install-dirs
+  prefix: @CWD@/test/data/.cabal-sandbox
   bindir: $prefix/bin
   libdir: $prefix/lib
   libsubdir: $arch-$os-$compiler/$pkgid

test/data/check-packageid/cabal.sandbox.config.in

@@ -4,15 +4,15 @@
 -- if you want to change the default settings for this sandbox.
 
 
-local-repo: /home/me/work/ghc-mod/test/data/.cabal-sandbox/packages
-logs-dir: /home/me/work/ghc-mod/test/data/.cabal-sandbox/logs
-world-file: /home/me/work/ghc-mod/test/data/.cabal-sandbox/world
+local-repo: @CWD@/test/data/.cabal-sandbox/packages
+logs-dir: @CWD@/test/data/.cabal-sandbox/logs
+world-file: @CWD@/test/data/.cabal-sandbox/world
 user-install: False
 package-db: test/data/check-packageid/.cabal-sandbox/i386-osx-ghc-7.6.3-packages.conf.d
-build-summary: /home/me/work/ghc-mod/test/data/.cabal-sandbox/logs/build.log
+build-summary: @CWD@/test/data/.cabal-sandbox/logs/build.log
 
-install-dirs 
-  prefix: /home/me/work/ghc-mod/test/data/.cabal-sandbox
+install-dirs
+  prefix: @CWD@/test/data/.cabal-sandbox
   bindir: $prefix/bin
   libdir: $prefix/lib
   libsubdir: $arch-$os-$compiler/$pkgid