Show "Add dependency" Code Action only if there is no `package.yaml` file

[fendor]

If the project is a stack project, there might be a package.yaml file. Adding a dependency to the generated .cabal file is a bad idea, as it will lead to annoying behaviour: Stack will no longer re-generate the .cabal file, so changes to package.yaml are not propagated to the .cabal file any more.

solution:

When looking for the .cabal file in the filesystem, check there is no package.yaml file next to it.

[ASnd-dev]

We (@JMoss-dev and I) are working on this issue.

[fendor]

Closed by #4422