[Feature request] yarn 2 support

[Reon90]

check-peer-dependencies --yarn --install
Usage Error: Couldn't find a script named "upgrade".

[christopherthielen]

I don't think this library will ever truly support yarn 2 because it relies on node_module traversals. I have a branch that detects yarn2 and uses yarn up instead of yarn upgrade but I'm sure this will only work if you use node_modules resolver in yarn 2. Try it out, and if it works I'll merge #40

[Chaz0r]

Fyi: yarn up should work. I tried this package on yarn 2 and I use pnp. I got the error that yarn upgrade does not exist. So i simply added a script in my package json called "upgrade" which simply runs "yarn up".
In that case yarn upgrade exists and executes yarn up. Worked perfectly fine.