getdeps: windows: avoid expanding drive letter substitutions
getdeps: windows: avoid expanding drive letter substitutions

Summary:
We use drive letter substitutions to workaround Windows
filename length limitations.  On my personal Windows system running
python 3.8.2 realpath inside an ssh session manages to resolve to
the full filename which causes the boost build to fail.

Avoid that!

Reviewed By: chadaustin

Differential Revision: D21507851

fbshipit-source-id: 1220c1c85d2124ddc51f42cefff2ce00e10c55c9

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift