Introduce manifest format using git submodules

If a manifest top level directory contains '.gitmodules' we now
assume this is a git module format manifest and switch to using
that code, rather than the legacy XML based manifest.

At the same time, we move the bare repository for a project from
$TOP/.repo/projects/$REPO_PATH.git to be $REPO_NAME.git instead.
This makes it easier for us to later support a repo init from an
existing work tree, as we can more accurately predict the path of
the project's repository in the workspace.  It also means that the
$TOP/.repo/projects/ directory is layed out like a mirror would be.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/manifest_loader.py b/manifest_loader.py
index 1ce1c1f..467cb42 100644
--- a/manifest_loader.py
+++ b/manifest_loader.py
@@ -13,11 +13,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from manifest_submodule import SubmoduleManifest
 from manifest_xml import XmlManifest
 
 def ParseManifest(repodir, type=None):
   if type:
     return type(repodir)
+  if SubmoduleManifest.Is(repodir):
+    return SubmoduleManifest(repodir)
   return XmlManifest(repodir)
 
 _manifest = None