Add the username external id when importing GitHub accounts

Gerrit uses now the username external id in addition to the
legacy gerrit external id scheme.

Change-Id: I81a7e2aff2884b27804dfa9dfca114cad9dcea47

github-plugin/src/main/java/com/google/gerrit/server/account/AccountImporter.java

diff --git a/github-plugin/src/main/java/com/google/gerrit/server/account/AccountImporter.java b/github-plugin/src/main/java/com/google/gerrit/server/account/AccountImporter.java
index a8f8155..549c560 100644
--- a/github-plugin/src/main/java/com/google/gerrit/server/account/AccountImporter.java
+++ b/github-plugin/src/main/java/com/google/gerrit/server/account/AccountImporter.java
@@ -43,6 +43,7 @@
     List<ExternalId> extIds = new ArrayList<>();
     extIds.add(ExternalId.createEmail(id, email));
     extIds.add(ExternalId.create(ExternalId.SCHEME_GERRIT, login, id));
+    extIds.add(ExternalId.create(ExternalId.SCHEME_USERNAME, login, id));
     AccountState accountUpdate =
         accountsUpdateProvider
             .get()