[Bug] yarn breaks with relative --cwd option

[kherock]

• I'd be willing to implement a fix

Describe the bug

Certain yarn 2.x subcommands seem to break when passing the --cwd option.

To Reproduce

Given the following hierarchy:

yarntest/
┣ .yarn/
┃ ┗ releases/
┃   ┗ yarn-berry.js
┣ subdir/
┃ ┗ workspace-b/
┃   ┗ package.json
┣ workspace-a/
┃ ┗ package.json
┣ .pnp.js
┣ .yarnrc.yml
┣ package.json
┗ yarn.lock

Attempting to pack each workspace from the the project root returns an error:

$ yarn --cwd workspace-a pack
Internal Error: ENOENT: no such file or directory, lstat '/home/user/yarntest/workspace-a/workspace-a'
    at Object.realpathSync (fs.js:1529:7)
    at /home/user/yarntest/.yarn/releases/yarn-berry.js:24:103298
    at async n (/home/user/yarntest/.yarn/releases/yarn-berry.js:24:102905)

$ yarn --cwd subdir/workspace-b pack
Internal Error: ENOENT: no such file or directory, lstat '/home/user/yarntest/subdir/workspace-b/subdir'
    at Object.realpathSync (fs.js:1529:7)
    at /home/user/yarntest/.yarn/releases/yarn-berry.js:24:103298
    at async n (/home/user/yarntest/.yarn/releases/yarn-berry.js:24:102905)

Environment if relevant (please complete the following information):

• OS: Linux
• Node version: 12.15.0
• Yarn version: 2.0.0-rc.29

Additional context

Passing an absolute path succeeds, but only when the subdirectory is a workspace folder (which is
a separate issue #705).