expose fewer internals in packages, for easier software reuse

- prometheus is now behind an interface, they aren't dependencies for the
  reusable components anymore.
- some dependencies have been inverted: instead of packages importing a main
  package to get configuration, the main package now sets configuration in
  these packages. that means fewer internals are pulled in.
- some functions now have new parameters for values that were retrieved from
  package "mox-".

smtpserver/server_test.go

@@ -1008,7 +1008,8 @@ func TestTLSReport(t *testing.T) {
 			tcheck(t, xerr, "write msg")
 			msg := msgb.String()
 
-			headers, xerr := dkim.Sign(ctxbg, pkglog.Logger, "remote", dns.Domain{ASCII: "example.org"}, dkimConf, false, strings.NewReader(msg))
+			selectors := mox.DKIMSelectors(dkimConf)
+			headers, xerr := dkim.Sign(ctxbg, pkglog.Logger, "remote", dns.Domain{ASCII: "example.org"}, selectors, false, strings.NewReader(msg))
 			tcheck(t, xerr, "dkim sign")
 			msg = headers + msg