Plugin execution contributed by lifecycle mapping gets lost when same goal is bound multiple times

When a plugin goal is implicitly bound twice from lifecycle mappings, the two goal executions get the same id and clash. This clash can eventually lead to all but one goal execution being lost (during model merging).

Originally reported as TYCHO-546 where org.apache.maven.plugins:maven-clean-plugin:clean is bound once for the clean lifecycle (from Maven core) and once for the default lifecycle (from the build extension), both times using the execution id default-clean.