Rename extraVirtualAliases to aliases and update description

The extra and virtual parts are redundant and Postfix specific and not
at all required. Compare forwards for example.

tests/external.nix

@@ -85,7 +85,7 @@
             };
           };
 
-          extraVirtualAliases = {
+          aliases = {
             "single-alias@example.com" = "user1@example.com";
             "multi-alias@example.com" = [
               "user1@example.com"
@@ -494,7 +494,7 @@
           # if this succeeds, it means that user1 received the mail that was intended for chuck.
           client.fail("fetchmail --nosslcertck -v")
 
-      with subtest("extraVirtualAliases"):
+      with subtest("Test sending from alias address (mailserver.aliases)"):
           client.execute("rm ~/mail/*")
           # send email from single-alias to user1
           client.succeed(

tests/ldap.nix

@@ -127,7 +127,7 @@ in
           localDnsResolver = false;
           indexDir = "/var/lib/dovecot/indices";
 
-          extraVirtualAliases = {
+          aliases = {
             # Steal frank@example.com from LDAP user frank
             "frank@example.com" = "mallory@example.com";
           };