Improvments to checking for conflicts among tag numbers and names:
1) Do not consider elements of a nested "extend" when checking for conflicts
   with "reserved" or "extensions" declarations.
2) Do not report more than one conflict error per element.
3) Consider elements of groups when checking for field-number conflicts.

Change-Id: Iee85fd7b16ef79686984f1f20ee9b22053577dd6