Ensure that pkg_resources.working_set (.entries) load lazily

[Alphadelta14]

Summary of changes

Speed up how long it takes to import pkg_resources by deferring load of WorkingSet.entries until they are needed (for e.g., iter_entry_points)

Importing pkg_resources requires a full traversal of all sys.paths and loading all contextual metadata for all packages and this can be quite slow under many many circumstances.

Closes pypa#510
Closes pypa#711
Closes pypa#926

Unlikely to pursue upstream merge as this targets both Python 2.7 and 3.7.

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details

[thomasaarholt]

Having just gone down the rabbit hole of looking as to why pkg_resources is slow to import, I arrived at this. I observe that you've created this PR to your own repo. Is this intentional, or were you intending to submit it to pypa/setuptools? I would be very interested in seeing pkg_resources imports sped up :)

[Alphadelta14]

@thomasaarholt I needed fixes for Python 2, so that's why I didn't direct upstream. But I guess with interest i could switch targets to there.
I was a bit discourged since pypa#510 was closed as WontFix though.

[thomasaarholt]

I'd argue that that issue went on for too long in a slightly unhealthy manner.

If your code does either load things lazily or speed it up in some manner, I'd imagine there would be people glad to see it.