Merge branch 'mplegendre-multi_pkgsrc_roots' into develop

- This moves var/spack/packages to var/spack/repos/builtin/packages.

- Packages that did not exist in the source branch, or were changed in
  develop, were moved into var/spack/repos/builtin/packages as part of
  the integration.

Conflicts:
	lib/spack/spack/test/unit_install.py
	var/spack/repos/builtin/packages/clang/package.py

bin/spack

@@ -140,8 +140,8 @@ def main():
 
     spack.spack_working_dir = working_dir
     if args.mock:
-        from spack.packages import PackageDB
-        spack.db = PackageDB(spack.mock_packages_path)
+        from spack.repository import RepoPath
+        spack.repo.swap(RepoPath(spack.mock_packages_path))
 
     # If the user asked for it, don't check ssl certs.
     if args.insecure: