Improve backend plugin ergonomics.

This adds support for omitting a backend entrypoint where
no backend plugin is provided and also improves import
errors by wrapping and giving the full module path that was
not found.

Test coverage is also added.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/base:all

Bugs closed: 666

Reviewed at https://rbcommons.com/s/twitter/r/666/
Simplify the JvmTarget resources property.

Resources are just another form of dependency as demonstrated
by the traversable_dependency_specs implementation.  They all
get resolved as dependencies during BUILD parsing so we just need
to look at our direct dependencies that are Resources target to
implement the resources property.  This has the side-benefit that the
standard dependency injection primitives for stitching synthetic targets
into a BUILD graph can be used to stitch in new Resources dependencies.

Testing Done:
$ ./build-support/bin/ci.sh

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/29898110

Bugs closed: 344, 346

Reviewed at https://rbcommons.com/s/twitter/r/673/
doc python_req things

give python_requirements_library a docstring

tag python_requirements with @manual so it shows up in BUILD dictionary

add example to python_requirements with a "this gives you something like..."

Bugs closed: 292

Reviewed at https://rbcommons.com/s/twitter/r/629/
Upgrade default setuptools to 5.4.1

Testing Done:
CI passed.  Looking at https://pypi.python.org/pypi/setuptools , there appear to be no expected breaking changes between our current setuptools (5.0) and this one.  We want this upgrade because of the bugfix in 5.1, and the other bug fixes since then seem good to pull in.

Reviewed at https://rbcommons.com/s/twitter/r/674/
Patch around unicode dup detection failure.

I'm not super happy with this solution and I think it deserves a closer look, but since the thread for this fizzled out, this is the patch we've been running internally to unblock ourselves.  Thread for reference: https://groups.google.com/forum/#!searchin/pants-devel/unicode$20decode/pants-devel/tx4NHhxX1fI/VRP0fzzG4C4J

Testing Done:
CI passed

Reviewed at https://rbcommons.com/s/twitter/r/615/
Add manifest parser to AndroidTarget.

This removes the need to enter target_sdk and package variables
for every BUILD file in the package. It was always on the TODO,
but as I am prepping the codegen tasks, it seemed time. This removes
several params from the targets, docstrings are updated accordingly.

Also added BadManifestError to catch malformed or incomplete manifests.
These errors would eventually cause problems in the compilation
process, but since we are parsing the manifest here, uncaught
exceptions would appear to be pants-related problems, as opposed
to an issue with the user's Android project. So I added an exception
to catch the issue.

Testing Done:
ci.sh passed locally.
Travis passed [0].

There will be some further integration tests of this functionality when I push up a
separate patch this weekend comprised of the aapt_gen class and its test coverage.

[0] https://travis-ci.org/pantsbuild/pants/builds/29720531

Reviewed at: https://rbcommons.com/s/twitter/r/668/
Fix relativity of antlr sources

This fixes an issue where CacheValidator's source hashing was failing on
the synthetic targets generated by AntlrGen, since the assumed source
paths mismatched the actual generated paths.

- Adds an integration test for antlr4 codegen
- Uses sources_relative_to_source_root for the assumed output sources,
  and fixes the synthetic source root for the antlr output. We can't
  control antlr's output relative paths very tightly, so we can't use
  the output directory directly as a sourceroot.
Lock globs into rootdir and below

This avoids the situation where a user adds a sources=globs('../*.scala'), causing it to be silently ignored but causes issues later.

Reviewed here: https://rbcommons.com/s/twitter/r/686/
Closes: https://github.com/pantsbuild/pants/issues/348
Pluralize product_type of Task to product_types
doc: Python doc, don't use twitter/commons as example

Back when Pants' source code lived in twitter/commons, it made sense to use example code from there. Nowadays, not so much. Use Pants' own src/python/example instead.

Instead of having sample code in the doc, "include" from src/python/example; that's easier to keep correct.

Testing Done:
build-support/bin/publish_docs.sh

Reviewed at https://rbcommons.com/s/twitter/r/654/
Stop pointing twitter/commons users at the "old python" docs

Reviewed at https://rbcommons.com/s/twitter/r/693/
make build_configuration test happy on OSX, not just linux

"Looks like we need to toss in a realpath somewhere in these tests" was true

Also a couple of missing deps reared their head when I ran this test on its own

Testing Done:
./pants goal test tests/python/pants_test/base:build_configuration

./build-support/bin/ci.sh # on my mac

Linux still happy
https://travis-ci.org/pantsbuild/pants/builds/30092011

Reviewed at https://rbcommons.com/s/twitter/r/694/
Merge branch 'master' into new_flags2
WIP