Don't re-download packages on every build

[rvagg]

From #160:

Don't re-download packages for every build: a little like npm, it should be aware of what you have in node_modules/ and not download the same package every time you rebuild.

This is already working in 1.0-wip and I'm using it in my build-process already and it's brilliant because Ender runs so much quicker. However, my feeling is that it's not quite right with package versions and tags, I wouldn't mind having feedback from others who have tried this out.

This ticket is a reminder to further investigate the implementation and see if it can be improved before a 1.0 release.

[rvagg]

this is in 1.0, there is a --force-install to override it.

[rvagg]

BTW, I'm leaving this open for discussion as I'm not 100% convinced this is the right approach or that the current implementation gets it right.

Part of the problem is that we're not caring enough about versions, which we should. npm has a cache already but each time you call install it has to check for the latest version so it can mean a build takes forever. The primary objective here is to make builds fast where they can be so it's an easy decision to include ender in your build chain.