Pass platform for SourceTranslator

[thesamet]

Similarly to WheelTranslator and EggTranslator, pass the platform to SourceTranslator. Without it, SourceTranslator takes the current platform.

[kwlzn]

@thesamet what's the point of passing this? afaik, sdists are compatible with all platforms - so the compatibility check is moot. and since there's no inherent cross-compilation ability, specifying a platform that isn't the current platform for SourceTranslator strikes me as odd.

[thesamet]

I am actually implementing cross compilation: I created a shell script that runs Python 2 inside a docker container and providing that as an interpreter to pants (and hence to pex). So for me, current platform is "macosx-10.11-intel" but the target platform is "linux-x86_64". The source translator is invoking bdist on the source targets and creates a linux-specific wheel. Then, the compatibility check here https://github.com/pantsbuild/pex/blob/master/pex/translator.py#L114 fails without this PR, since self._platform would be OS X and not Linux.

[kwlzn]

@thesamet ah, gotcha.