feat(nodeenv): install local packages from a requirements file

[ekalinin]

Closes #206.

Problem

-r/--requirements took a single value: no action='append', no nargs. Repeating the flag silently kept the last file, with no error and no merge:

-r global-req.txt -r local-req.txt  ->  requirements = 'local-req.txt'

And install_packages() always installed globally (npm install -g), with no option anywhere in --help asking for a local install. The gap is visible from the other side too: freeze -l was added for #71 and is documented in the README, so nodeenv could produce a list of locally installed packages but not install one. The file it printed, fed back through -r, installed globally.

Change

• -r/--requirements may be given more than once; files are read in the order given.
• New --local-requirements, also repeatable, installs the packages of a file without -g, into node_modules of the directory nodeenv was invoked from. That is the directory freeze -l was run from, and the one node resolves from when the project runs.
• Global packages are installed first, then the local ones.

$ cd myproject
$ nodeenv --requirements=global-req.txt --local-requirements=local-req.txt env
myproject/
├── env/
│   └── lib/node_modules/   <- from -r
└── node_modules/           <- from --local-requirements

-r file behaves exactly as before, and requirements is not a Config attribute, so ~/.nodeenvrc, tox.ini and setup.cfg are unaffected. The --update path needed no change: it calls install_packages() directly, which now handles both lists.

The npm < 1.0.0 branch (npm install + npm activate) is left alone - it has no -g to drop, so a local file is still installed the old way there. Noted in CHANGES.

Tests

install_packages() had no test at all. Four new ones cover the repeated -r, the new flag, and the generated npm commands (-g present for global files in file order, absent for local ones, locals last).

Verified on a real environment as well: nodeenv --node=system -r glob.txt --local-requirements loc.txt env puts left-pad in env/lib/node_modules and mime in ./node_modules; freeze and freeze -l in that environment then print the two lists back.