Allow packages key

[cjmartian]

Allows for the use of 'packages' as an alias for 'dependencies' as requested here:
#194

[cjmartian]

Just ran python scripts/run_tests.py locally on my mac:
anaconda_project/requirements_registry/providers/test/test_conda_env.py::test_prepare_and_unprepare_project_scoped_env PASSED
python --version Python 3.7.3

The travis-ci failure is on mac build target 3.7

[mcg1969]

@cjmartian, sorry this took so long to look at but I have some bad news—this isn't actually what was asked for in #194. What that issue is requesting is for the syntax for anaconda-project.yml itself to be altered, so that dependencies: can be freely used wherever packages: is currently expected. To see what this means, take a look at this file, referred to in #194:

https://github.com/pyviz-topics/examples/blob/master/bay_trimesh/anaconda-project.yml

Note the use of dependencies:, but also this &deps/*deps trick.

[cjmartian]

@mcg1969 This branch should have been named allow-dependencies-key sorry about that.

I tested that I can still run anaconda-project run with an anaconda-project.yml from the examples directory that has the key dependencies rather than packages and it seems to work.