Let a module-declared seat carry needs, so mail-notifier can need node-notifier (issue 802)

ADR 0220's dependency mechanism served compiled seats only and retired its one user with
node-resolver-config (commit f68521da). ADR 0378 S8 revives it for module-declared seats:
SeatDeclaration gains Needs, judged by the catalogue check (node scope only, a name that
resolves to a node seat somewhere in the catalogue, never itself), and DependsOn now takes
the catalogue so a claimed seat's needs are found wherever they are declared.

internal/catalogue/contribution_dependencies_test.go

@@ -34,7 +34,7 @@ func TestAContributionDependsOnTheSeatThatPlacesIt(t *testing.T) {
 		"an empty environment":      {empty, nil},
 	}
 	for name, c := range cases {
-		got := DependsOn(c.m)
+		got := DependsOn(nil, c.m)
 		if len(got) == 0 {
 			got = nil
 		}
@@ -118,7 +118,7 @@ func TestCodeForAPowerMomentDependsOnThePowerSeat(t *testing.T) {
 	for _, moment := range powerMoments {
 		m := mod("laptop", nil, nil, nil)
 		m.Shell = []ShellCode{{For: moment, Slot: "normal", Code: "true"}}
-		if got := DependsOn(m); !reflect.DeepEqual(got, []string{PowerSeat}) {
+		if got := DependsOn(nil, m); !reflect.DeepEqual(got, []string{PowerSeat}) {
 			t.Errorf("code for %s depends on %v, want %s", moment, got, PowerSeat)
 		}
 		if p := m.shellProblems(); len(p) != 0 {