Warn on package lock json

[jmorrell]

Summary

This is a first pass at resolving #5654 by adding a warning when npm lockfiles already exist in the directory when yarn is run

Test plan

• Added integration tests and fixtures that check to make sure warning is printed to the console
• Ran locally using yarn-local alias:

[jmorrell]

Ideally I'd like to make the warning message more prominent, but it's not clear what mechanism I'd use to do that.

Happy to iterate on any part of this

[imsnif]

This looks great.

We have accurate tests, it pops when it needs to (with an existing yarn.lock file, when creating a new yarn.lock file) and does not pop when it doesn't need to (when importing to a new yarn.lock file).
The messages are verbose and provide a clear action to be performed to remedy the situation.

[imsnif]

@jmorrell - while I agree that the message should be more prominent (I'd personally like to see this become an error that fails the install as a breaking change), there is currently some disagreement on this subject.
I don't think we have a facility to not fail the install while being more prominent than a warning.
I think the best way to proceed right now would be to merge this, and upon its release in the next version, try to see if it reduces the deployment errors you mentioned in #5654
If after a while we feel this has not made a big enough dent in them, we can consider more drastic measures. What do you think?

[jmorrell]

@imsnif I agree with everything you've said :)

[imsnif]

Great. Thanks for this patch!

[arcanis]

😍