use meta_get to get package manager name

[danielleadams]

It seems that the all package managers are being reported as npm, so the if statement here is not true: https://github.com/heroku/heroku-buildpack-nodejs/blob/master/bin/report#L107, and the reporting is wrong.

This pull request creates a method in lib/dependencies.sh to use in both bin/compile and bin/report to detect for a yarn.lock file (and subsequently use Yarn logic).