Fix to be usable with monorepos using Pub workspaces

[myConsciousness]

• Check if pubspec.lock exists before trying to read it
• Prevents crashes when running in monorepo environments
• Fixes issue PathNotFoundException: Cannot open file - pubspec.lock #85

[myConsciousness]

Hi @vkammerer, could you check this PR?

[vkammerer]

@myConsciousness I have good news for you and your PR.

@mafreud is going to be the new maintainer. As soon as he is granted access to the repo and to the pub.dev account, your PR will move forward.

[myConsciousness]

If you're struggling with the same error and this PR hasn't been merged yet, temporarily use the following fork as a workaround. This fix works perfectly in my monorepo project.

Add the following to your pubspec.yaml:

dev_dependencies:
  import_sorter:
    git:
      url: https://github.com/myConsciousness/import_sorter.git
      ref: fix/monorepo-pub-workspaces