feat(cli): Generate an empty lockfile when there are no dependencies

[jimmyhchan]

Summary

this reverts most of #3395 992b5c9 which appears only necessary to work around #3329 (requesting the yarn version with -v triggered install). As noted in #5839, yarn -v and yarn check no longer have install as a side effect and IMO, the original behavior where an empty lock file is generated as part of install is the correct behavior.

Many tools currently sniff for a yarn.lock file to determine if yarn is being used. It seems more consistent with yarn import which generates a lockfile even if the existing node_modules is empty.

fixes #5839

Test plan

yarn run test passes
yarn run lint passes
yarn-local install saves an empty lockfile
yarn-local import continues to save an empty lockfile
yarn-local -v will not trigger install and will not generate a lock file. correctly displays a version.
yarn-local check will not trigger install and will not generate a lock file

[rally25rs]

I like this change. IMO if there is a package.json and you run yarn install, an empty lockfile makes sense. @BYK thoughts?

Verified that yarn -h, yarn help, yarn check, yarn -v don't create the lockfile.

[BYK]

I really like this change but wanna hear what @arcanis and @imsnif thinks befor merging. Also cc @Daniel15 and @bestander. I'll merge from master in the meantime to kick off Node 10 builds.

[arcanis]

Yep, looks fine by me 👍