This fixes a race condition between UStaticMesh::GetPlatformStaticMeshRenderData calling GetPackage from a task thread to know if the package is already cooked for editor, while we are moving an HLOD mesh from an external actor package into a streaming level during cooking, resulting in a discrepency betwen the package RF_HasExternalPackage and the content of the external package hash map. The situation still exists, but we know that it's not going to cause problems in that specific case.
[REVIEW] [at]francis.hurteau
#rb francis.hurteau
#p4v-cherrypick 27150485
#localization none
#tests none
[CL 27391618 by jeanfrancois dube in 5.3 branch]
52341e23ce