Fail on atomic_directory work_dir collision. (#1905)

Previously, in the astronomically unlikely case that two (or more)
attempts at creating an atomic directory collided on their UUID4-based
work directory, the colliding attempts would all succeed and possibly
lead to a corrupt final target directory. Now work directory creation
fails loudly when the work directory already exists. Re-try of the Pex
command will ~always work in this case but we'll have a paper trail of
a UUID4 collision.

Fixes #1901

pex/common.py

@@ -466,7 +466,7 @@ def unlock():
             return
 
     try:
-        safe_mkdir(atomic_dir.work_dir)
+        os.makedirs(atomic_dir.work_dir)
         yield atomic_dir
         atomic_dir.finalize(source=source)
     finally: