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/seat_data_test.go

@@ -333,9 +333,9 @@ func TestAnOfferedBlockMakesNoDependencyOnTheBar(t *testing.T) {
 		block(`{"bar":"bottom","place":"status","shows":"battery"}`),
 		{Seat: HotkeysSeat, Kind: "trigger", Content: "x"},
 	}}
-	for _, seat := range DependsOn(power) {
+	for _, seat := range DependsOn(nil, power) {
 		if seat == BarSeat {
-			t.Fatalf("a block made the power module depend on the bar: %v", DependsOn(power))
+			t.Fatalf("a block made the power module depend on the bar: %v", DependsOn(nil, power))
 		}
 	}
 	// And on a machine with no bar it is placed nowhere, and nothing fails.