Add a yarn dependency source

[jonabc]

As found in #30 (comment)

The errors appear to be because my package.json file is actually intended for yarn, and uses some syntax that is not npm-compatible

Yarn's package.json files attempt to be compatible with NPM package.json files but can contain differences that cause the NPM dependency source to fail.

It looks like an indicator that yarn should be used and that npm shouldn't be used is the presence of a yarn.lock file.

[jonabc]

I took a look through the yarn CLI and unless I'm missing something, it's nowhere near as easy to enumerate all the data that we need as it is from npm.

I could use some help figuring out either

• how to enumerate the name, version, directory location (mandatory) and homepage, author (optional, but needed to have parity with the npm source)
• if it's possible to use npm cli commands with a yarn package.json

[krzysztof-pawlik-gat]

I've started a pr to have the path added to yarn list --json: yarnpkg/yarn#7462