Do not search for cross import when building swiftinterfaces. This is
uncessary because:
* The cross import modules are explicitly listed in the interface file
thus there is no need to walk the file system to discover more
cross imports.
* All cross imports are discovered by scanner already and the
dependencies are passed to the interface building job. There is no
need to search file system to find the module to consume.
This reduces the work needed when building interface and fix a bug that
the file system walk when caching is enabled can cause build failures on
windows platform.
489436ffab