clean up find_links=, tahoe-lafs/deps/ web cache

[warner]

Tasks to do:

• publish OS-X and windows wheels of all dependencies to ​https://tahoe-lafs.org/deps/
• change tahoe.cfg to point find_links= at that (rather than separate sdist/ and eggs/ directories) (NEVERMIND)
• get rid of the confusing mess of other tahoe-lafs.org dependencies URLs
• manually test an OS-X build without a compiler
• manually test a windows build without a compiler

[warner]

Daira pointed out that we should keep the old tahoe-lafs.org dependency directories around for now, for the sake of the previous release. We can deprecate them after 1.11.0 is out.

[warner]

I've removed the find_links= from tahoe.cfg because it was only getting used by commands that run setup.py directly (e.g. setup.py install or setup.py develop). Our instructions direct users to do pip install ., and pip doesn't look at find_links in setup.cfg .

Instead, to take advantage of our published wheels, users should do pip install --find-links=https://tahoe-lafs.org/deps/ .. I'm updating the mac/windows docs to suggest that, because it will help on those platforms. I'm not updating the linux docs for it, because it doesn't help very much (installing a wheel is marginally faster than installing a pure-python tarball, but not enough to compensate for the distraction of an extra argument).

Wheels are published to the URL above.

I tested a windows install with the --find-links= argument, and it ran quickly (and didn't appear to compile anything). The only windows instance I have access to (a GCE VM) has the compiler installed, so I don't have a way to test without a compiler, but I think eyeballing the lack of "building XYZ.." lines in the output is probably good enough.

[warner]

I think that's good.