lookup() searches for symbols in the given binding and the enclosing scopes,
while find() searches for symbols only in the given binding.

Main reason is that find-usages and refactoring broke completely.
Second reason is that concurrent modification was done on the
children-list while deleting/destructing.

This reverts commit b0b95f88.

Patch by: Bill King

Task-number: QTCREATORBUG-534
Reviewed-by: Thorbjørn Lindeijer <thorbjorn.lindeijer@nokia.com>

Encapsulate the details.

Null pointer reference in a case where the global namespace is
explicitly specified using ::

Task-number: QTCREATORBUG-351
Review-pending-by: Roberto Raggi