*: git.monogon.dev -> source.monogon.dev

This implements T882, setting our (virtual) GOPATH to source.monogon.dev
for this repository.

Test Plan: Refactor, CI only.

X-Origin-Diff: phab/D686
GitOrigin-RevId: c5e2309089948ffc3a98e68e2e0e1cbb157d3a36
diff --git a/metropolis/pkg/logtree/leveled.go b/metropolis/pkg/logtree/leveled.go
index c24357e..c0d2aff 100644
--- a/metropolis/pkg/logtree/leveled.go
+++ b/metropolis/pkg/logtree/leveled.go
@@ -19,7 +19,7 @@
 import (
 	"fmt"
 
-	apb "git.monogon.dev/source/nexantic.git/metropolis/proto/api"
+	apb "source.monogon.dev/metropolis/proto/api"
 )
 
 // LeveledLogger is a generic interface for glog-style logging. There are four hardcoded log severities, in increasing