smalltown -> metropolis

This pass removes all mentions of Smalltown, both from code and comments,
and replaces them with appropriate new terminology.

Test Plan: Refactor, covered by CI.

X-Origin-Diff: phab/D674
GitOrigin-RevId: 04a94d44ef07d46f7821530da5614daefe16d7ea
diff --git a/metropolis/proto/common/common.proto b/metropolis/proto/common/common.proto
index def91e8..4defcbe 100644
--- a/metropolis/proto/common/common.proto
+++ b/metropolis/proto/common/common.proto
@@ -15,7 +15,7 @@
 // limitations under the License.
 
 syntax = "proto3";
-package smalltown.core.proto.common;
+package metropolis.proto.common;
 option go_package = "git.monogon.dev/source/nexantic.git/metropolis/proto/common";
 
 enum TrustBackend {