pex resolver crashes when using local cache for certain transitive requirements

[wickman]

packages_from_requirement_cached doesn't check if everything in the existing= kwarg is local, which means it may attempt to os.stat a remote file in certain circumstances.

mba=pex=; pex -r pytest -r setuptools -r py==1.4.25 -o /tmp/pt.pex --cache-ttl=3600 -v -v -v -v -v
pex: Package cache hit (inexact): pytest        
pex: Package cache hit (inexact): setuptools        
pex: Package cache miss: py==1.4.25                 
pex: Resolving distributions :: Fetching https://pypi.python.org/packages/source/p/py/py-1.4.25.tar.gz#md5pex: Validated py-1.4.25.tar.gz (md5=1f0b84dab1681fc1bf07c0528e091efb)                                                                     
pex: Validated py-1.4.25.tar.gz (md5=1f0b84dab1681fc1bf07c0528e091efb)
pex: Resolving distributions :: Packaging py :: Installing /var/folders/rd/_tjz8zts3g14md1kmf38z6w80000gn/pex: Resolving distributions :: Packaging pytest                                                          pex: Resolving distributions :: Packaging pytest :: Installing /var/folders/rd/_tjz8zts3g14md1kmf38z6w8000pex: Resolving distributions :: Resolving py>=1.4.25                                                      Traceback (most recent call last):
  File "/Users/wickman/Local/bin/pex/.bootstrap/_pex/pex.py", line 272, in execute
    self.execute_entry(entry_point, args)
  File "/Users/wickman/Local/bin/pex/.bootstrap/_pex/pex.py", line 320, in execute_entry
    runner(entry_point)
  File "/Users/wickman/Local/bin/pex/.bootstrap/_pex/pex.py", line 343, in execute_pkg_resources
    runner()
  File "/Users/wickman/.pex/install/pex-0.8.0.dev0-py2.py3-none-any.whl.0d5be588022f0aad6de2b84b77a8a890c43ad942/pex-0.8.0.dev0-py2.py3-none-any.whl/pex/bin/pex.py", line 325, in main
    pex_builder = build_pex(args, options)
  File "/Users/wickman/.pex/install/pex-0.8.0.dev0-py2.py3-none-any.whl.0d5be588022f0aad6de2b84b77a8a890c43ad942/pex-0.8.0.dev0-py2.py3-none-any.whl/pex/bin/pex.py", line 303, in build_pex
    cache_ttl=options.cache_ttl)
  File "/Users/wickman/.pex/install/pex-0.8.0.dev0-py2.py3-none-any.whl.0d5be588022f0aad6de2b84b77a8a890c43ad942/pex-0.8.0.dev0-py2.py3-none-any.whl/pex/resolver.py", line 215, in resolve
    existing=distribution_set.get(requirement.key))
  File "/Users/wickman/.pex/install/pex-0.8.0.dev0-py2.py3-none-any.whl.0d5be588022f0aad6de2b84b77a8a890c43ad942/pex-0.8.0.dev0-py2.py3-none-any.whl/pex/resolver.py", line 94, in packages_from_requirement_cached
    packages = [package for package in packages if (now - os.path.getmtime(package.path)) < ttl]
  File "/Users/wickman/Python/CPython-2.7.8/lib/python2.7/genericpath.py", line 54, in getmtime
    return os.stat(filename).st_mtime
OSError: [Errno 2] No such file or directory: '/packages/source/p/py/py-1.4.25.tar.gz'