*: 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/proto/api/enrolment.proto b/metropolis/proto/api/enrolment.proto
index 3c9b862..cf66adb 100644
--- a/metropolis/proto/api/enrolment.proto
+++ b/metropolis/proto/api/enrolment.proto
@@ -16,7 +16,7 @@
 
 syntax = "proto3";
 package metropolis.proto.api;
-option go_package = "git.monogon.dev/source/nexantic.git/metropolis/proto/api";
+option go_package = "source.monogon.dev/metropolis/proto/api";
 
 // EnrolmentConfig is the single Metropolis node boot configuration file
 // contained in the ESP. It configures the way the node will start up (what