Fix Yarn detection when using workspaces, fix .yarnrc.yml loading

[srmagura]

Issues Addressed

1. Resolves npm erroneously used in Yarn Workspaces monorepo #1120.
2. Resolves an issue I noticed during implementation: if the packageFile option was passed, the code still looked in . (current directory) when checking for a yarn.lock. Now it starts looking in the directory of the package file. I believe this is the correct behavior.
3. Resolves the local .yarnrc.yml always being loaded from ./.yarnrc.yml, which would not work if the packageFile option is passed or we are in a non-root workspace directory.
4. Resolves the user .yarnrc.yml never being loaded. The code used fs.existsSync('~/.yarnrc.yml'), but this does not interpret ~ as the user's home directory.

Notes

• I put the new test file in the same directory as the file being tested. This is my preferred pattern (rationale here), but I know this differs from the organization used for the other test files in the repo. I'm happy to move the test file if you tell me where it should go.
• I added a mocha script to enable running a single test file, e.g. npm run mocha -- src/lib/determinePackageManager.test.ts
• If you'd like to test fixes 1 and 2 against a "real" repository that uses Yarn workspaces, you can clone https://github.com/srmagura/npm-check-updates-1120-test. Fixes 3 and 4 are easiest to test if you have access to a private npm registry, e.g. the registry for FontAwesome Pro.
• I tested .npmrc loading with npm workspaces and it worked, so that doesn't need to be fixed. 🎉

[srmagura]

Converting to draft because, I realized the logic for finding the .npmrc/.yarnrc.yml is probably also broken...

[raineorshine]

• I put the new test file in the same directory as the file being tested. This is my preferred pattern (rationale here), but I know this differs from the organization used for the other test files in the repo. I'm happy to move the test file if you tell me where it should go.

I appreciate the point, though let's stick with a separate test folder to be consistent with the project's conventions.

[raineorshine]

Looks great!

[srmagura]

Thanks for the feedback, should be good now :)

[srmagura]

Thanks for merging. I checked out your changes, it all looked good except, I find nested ternaries to be hard to read. I guess not everyone feels that way though. No need to change 👍

[raineorshine]

Thanks for your contribution! I'll release it shortly.

Thanks for merging. I checked out your changes, it all looked good except, I find nested ternaries to be hard to read. I guess not everyone feels that way though. No need to change 👍

👏👏👏 preferences 🙃🙃🙃