Convert Project.NameKey to an immutable type

Follows the pattern of other key type refactorings, adding a static
factory method. However, unlike the other key types, this one can't be
an @AutoValue, because it has other subclasses and specific requirements
around the implementation of equals. So we're stuck with boilerplate; at
least it's only one field.

See I6982fb24 for context.

Change-Id: Ic46c4a357f4ee77f18e54e87ca97af6e834834b6

java/com/google/gerrit/server/util/git/SubmoduleSectionParser.java

@@ -133,7 +133,7 @@ public class SubmoduleSectionParser {
                   0, //
                   project.length() - Constants.DOT_GIT_EXT.length());
         }
-        Project.NameKey projectKey = new Project.NameKey(project);
+        Project.NameKey projectKey = Project.nameKey(project);
         return new SubmoduleSubscription(
             superProjectBranch, Branch.nameKey(projectKey, branch), path);
       }