ZSTD-4160 [436249] First call to isDisabled() always returns false

A call to DisabledResourceManager.isDisabled() will always return false
if the project is not loaded yet. This is because on line 124 it is
checked if the _disabled set contains the IResource object itself
instead of the IPath of the resource.

Signed-off-by: Kaloyan Raev <kaloyan.r@zend.com>