Skip broken vendor symlink rather than returning an error

[wolkykim]

What does this do / why do we need it?

Skip broken vendor symlink rather than returning an error

What should your reviewer look out for in this PR?

When one of 3rd party packages has a broken vendor symlink, it fails whole ensure process.

Current logic behaves:

• remove a vendor symlink linked to a valid directory
• leave a vendor symlink linked to a valid file
• return error if a vendor symlink is broken (linked file doesn't exist)

I think this behavior is not intended and it seems to be that it's better to skip the check except the case it's linked to a valid directory as it's harmless rather than returning an error causing the whole process stop.

Expected/New behavior:

• remove a vendor symlink linked to a valid directory
• leave rest of the case(linked to a file or broken) as they are not harmful rather than returning an error

Do you need help or clarification on anything?

no

Which issue(s) does this PR fix?

This is a new behavior introduced in v0.3.1 with its improved type checking
Since this is a breaking change, some users migrating from 0.3.0 could experience this issue.
The main problem is that when this happens on some 3rd party libraries that you don't have the control, you just have to downgrade and use v0.3.0

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot.
• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[wolkykim]

I signed it!

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[wolkykim]

please ignore this PR and see #1191 instead. Closing