Skip install script if already installed

[yyx990803]

The install script sometimes throws error on subsequent yarn installs since yarn seems to invoke install scripts when it is shuffling things around. It also causes issue for CI where the node_modules directory is cached across builds.

[evanw]

Ah that's annoying. I use npm, not yarn, so I've only been testing with that. You don't happen to have a repro, do you?

I'm wary that this change won't fix the problem. Right after the call to installPackage(), the install script then does a fs.renameSync() of a file inside the installation directory to move it into place. I'm guessing that call will fail if it's already been moved. So I suspect that you'll also have to tell the caller to return early too. But it'd be great to have a repro to double-check that this is correct.

[evanw]

Actually, since it sounds like the postinstall script just needs to be idempotent, this should be something I can test. I'll merge this and fix it up so it works.