Fix concurrent loading of the same qmldir from different scripts

QQmlQmldirData keeps a pointer to a QQmlScript::Import, and an integer
priority. Each Blob that is waiting on it was setting its own import
and priority even though the QQmlQmldirData itself was shared. This
resulted in whichever one began loading last succeeding to load,
and the rest failing. This change instead stores the import and
priority data per-dependent Blob

Task-number: QTBUG-30469

Change-Id: Id8b646f820c272b4ceb113b024c4fe3c0d1b8465