Prefer wheels during plugin install

[stuhood]

Pants has sprouted a dependency on a package that is most easily installed via a whl on pypi (openssl for requests[security]).

But currently the plugin installation infrastructure does not enable usage of WheelPackage due to a defensive avoidance of assuming that wheel is installed in an environment that pants is being loaded from:

pants/src/python/pants/init/plugin_resolver.py

Lines 79 to 82 in e0d5108

	# When bootstrapping plugins without the full pants python backend machinery in-play, we are not
	# guaranteed a properly initialized interpreter with wheel support so we enforce eggs only for
	# bdists with this custom precedence.
	precedence = (EggPackage, SourcePackage)

As far as @kwlzn or I can tell, wheel should always be on the PYTHONPATH, as pants itself depends on it: it's not clear that it actually needs to be "installed" on the PATH as well.

cc @jsirois for any context he might have on that comment.

[jsirois]

I'm still not fully sure about the comment. That said, adding WheelPackage to the head of the precedence list (or deleting the custom precedence argument), leads to a new error - pkg_resources from setuptools==30.0.0, which Pants uses, does not know how to find plugins except if they reside in eggs. Newer versions setuptools>=34.2.0 can do this however (http://setuptools.readthedocs.io/en/latest/history.html#v34-2-0).

[jsirois]

OK, newer setuptools cannot in fact deal with wheels in raw form, but adding some plugin resolution code to install wheels and activate them does the trick. PR soon.

[kwlzn]

thanks @jsirois !