Test the resolver file as the uplink's, and refuse a second writer of a fact's path (hq ADR 0223)

The catalogue moves /etc/resolv.conf from resolv-conf to the three uplink
modules. A rendered fact was not compared with other modules' paths, so two
modules could each write the resolver file on one machine, the last winning
every apply; a fact's path now counts as its module's.

internal/catalogue/resolve.go

@@ -939,6 +939,23 @@ func checkResources(modules []Manifest) []string {
 				}
 			}
 		}
+		// **A file the mesh renders for a module is that module's path too** (novox/hq ADR 0223). The
+		// machine's resolver file is a fact the uplink's holder asks for, and a second module asking
+		// for it, or declaring it, would have the host write one path twice in every apply, the last
+		// one winning. A fact under the operator's home is placed per account and compared nowhere.
+		for _, name := range sortedFacts(m.Facts) {
+			fact := m.Facts[name]
+			if fact.Home || !strings.HasPrefix(fact.Path, "/") {
+				continue
+			}
+			key := "path " + fact.Path
+			if other, taken := owner[key]; taken && other != m.Module {
+				problems = append(problems, fmt.Sprintf(
+					"%s and %s both declare the path %q", other, m.Module, fact.Path))
+			}
+			owner[key] = m.Module
+			ownedPath[fact.Path] = m.Module
+		}
 	}
 
 	// An accessed path is the operator's, so no module may declare it as one of its own