Minor fixes.

- Bug fix - use .Errorf() instead of .Error():
  Before this change, an error message for failed tests in files/set_test.go
  wrongly tried to use format strings in a call to .Error()

- Change event constants to be of type EventType rather than untyped integers.

- Slightly relax the locking for registry cache lookups:
  For read access to Discover.registry we only need to take a read lock.

- Some cosmetic fixes.

internal/files/set_test.go

@@ -858,7 +858,8 @@ func TestLongPath(t *testing.T) {
 		t.Fatalf("Incorrect len %d != 1 for global list", l)
 	}
 	if gf[0].Name != local[0].Name {
-		t.Error("Incorrect long filename;\n%q !=\n%q", gf[0].Name, local[0].Name)
+		t.Errorf("Incorrect long filename;\n%q !=\n%q",
+			gf[0].Name, local[0].Name)
 	}
 }