Fix corner case in cached dependency resolution

[butlern]

Ref: #178

The error is as follows:

1. Two resolvables, one pinned, the other not.
2. First pex run with clean cache works.
3. Change the pinned version.
4. Second pex run fails with Unsatisfiable.

The problem is that the non-pinned resolvable locates the cached dep
and only returns it. Since that version doesn't match the pinned
version, we fail. This fixes that behavior by returning both the
cached version and all uncached pypi packages for unpinned
resolvables.

We change the pinned version to test the use case where you have a
cached version of a dep and then you transitively pull in another
requirement that either has a tighter bound or a pin on a version not
found in the cache.

[kwlzn]

lgtm w/ one minor thought. thanks for fixing this bug!

[kwlzn]

assuming packages == [] in the non-cached case, how would you feel about killing the slight bit of super().package_iterator() call duplication here (lines 255 + 258) by always returning a combined iterable that includes packages - whether empty or not - in the main return statement?

e.g.

  if self.__cache_ttl:
    packages = ...

return itertools.chain(packages, super(...).package_iterator(...))

[butlern]

So something like this?

  def package_iterator(self, resolvable, existing=None):
    iterator = Iterator(fetchers=[Fetcher([self.__cache])])
    packages = self.filter_packages_by_interpreter(
        resolvable.compatible(iterator), self._interpreter, self._platform)

    if packages:
      if resolvable.exact:
        return packages

      if self.__cache_ttl:
        packages = self.filter_packages_by_ttl(packages, self.__cache_ttl)
      else:
        packages = []

    return itertools.chain(packages, super(CachingResolver, self).package_iterator(resolvable, existing=existing))

[kwlzn]

well, you could go a step further and guard the population of packages altogether by checking for the conditions in which it'd be used - was sort of imagining something along the lines of:

  def package_iterator(self, resolvable, existing=None):
    packages = []
    if self.__cache_ttl or resolvable.exact:
      iterator = Iterator(fetchers=[Fetcher([self.__cache])])
      packages = self.filter_packages_by_interpreter(
        resolvable.compatible(iterator),
        self._interpreter,
        self._platform
      )

      if packages:
        if resolvable.exact:
          return packages

        if self.__cache_ttl:
          packages = self.filter_packages_by_ttl(packages, self.__cache_ttl)

    return itertools.chain(
      packages,
      super(CachingResolver, self).package_iterator(resolvable, existing=existing)
    )

but I'm probably just splitting hairs at this point - fine with any of the above.

[butlern]

Gotcha, yah that looks good to me. I'll push up the change. For now I'll do a separate autosquashable commit unless you'd prefer it be a separate commit. Local tests run fine with that change btw :)

[kwlzn]

merged - thanks for the PR @butlern !