setup_repo: now only 98% impossible

This page was full of doom and gloom: Want to start using Pants? Might as well just give up.

Walk back some of that. Point out some files that you need to copy+adjust.

This probably still doesn't capture everything folks need, but it could be a step in the right direction.

rm the howto_customize page. For a while, I thought such a page would be useful... but its material all falls into setup_repo and howto_plugin.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/40251344

Bugs closed: 632, 726, 776

Reviewed at https://rbcommons.com/s/twitter/r/1295/
Remove a bunch of setup_parser plumbing from goal/task.

reflect.py relied on the old options system via setup_parser, so
it's been broken for a while, ever since options migration began.
Fixed that by making it read from the new options.

Testing Done:
Manually ran the doc generator and inspected the resulting docs.

CI passed.

Reviewed at https://rbcommons.com/s/twitter/r/1297/
Remove some stray calls to setup_parser.

One is in help.py which is completely unused, so now deleted.

The other was in the new help printer.

Testing Done:
Verified that --help works.

CI passed.

Reviewed at https://rbcommons.com/s/twitter/r/1300/
docs: mention mvn2pants

while I was in there: setup_repo had a paragraph about top-dir BUILD files for config. LOL, nowadays, that config has moved into plugins. So pull that out, and re-jigger examples of BUILD.foo files.

Reviewed at https://rbcommons.com/s/twitter/r/1301/
Remove a superfluous test.

It only tests that Goal.parse_args works, but that function is
about to be killed itself.

Reviewed at https://rbcommons.com/s/twitter/r/1304/
Move cache logging to debug

Can get a tad noisy with several tasks all using setting up multiple caches.

Testing Done: https://travis-ci.org/pantsbuild/pants/builds/40392152

Bugs closed: 783

Reviewed at https://rbcommons.com/s/twitter/r/1310/
make it clear: you can only download those pexes because you previously uploaded 'em.

they don't appear by magic.

Reviewed at https://rbcommons.com/s/twitter/r/1296/
Ensure that the new options system reports goals in the right order.

We sometimes rely on this, e.g., when specifying "clean-all compile".

Testing Done:
option unittests pass. CI passed when run with https://rbcommons.com/s/twitter/r/1309/

Reviewed at https://rbcommons.com/s/twitter/r/1308/
Switch option parsing for goals to the new parser.

Relies on https://rbcommons.com/s/twitter/r/1308/.

Testing Done:
CI passed when run with https://rbcommons.com/s/twitter/r/1308/.

Reviewed at https://rbcommons.com/s/twitter/r/1309/