[Question] Workspaces support

[sirgalleto]

Hi, I'm opening this PR because I see an opportunity to add support to workspaces/monorepos/multi-package repositories. You can find the npm RFC of the feature here

When you're working with workspaces, is common that each workspace has its own test suit, and maybe a custom configuration.

As far as I understand, the current plugin configuration

1. Gets the coverage-final.json file.
2. Gets the updated files.
3. Matches each file path with the info in the coverage-final.json

But in the case of a workspace, having multiple tests configuration and coverage-final.json or even packages with no tests, you end up with a non-reliable coverage report.

I think we can tweak this for workspaces adding multiple code coverage configurations with a package path to isolate the code-coverage analysis under that path

for example:

codeCoverage([
  {
    package: 'packages/package-a'
    ignoreCoveragePattern: ['.json', '.yml'],
    coverageFilesPath: 'packages/package-a/coverage/coverage-final.json'
  }, 
  {
    package: 'packages/package-b'
    ignoreCoveragePattern: ['.json', '.yml'],
    coverageFilesPath: 'packages/package-b/coverage/coverage-final.json'
  }
])

What do you think? I'm willing to take an action and code this implementation.

[Spoutnik97]

What I understand is that files modified in package-a are in the code-coverage analysis of the package-b whereas the package-b coverage-file.json does not include package-a files. So the coverage for these files are 0%. Do I understand well your problem ?

I think that test configurations should not influence the code-coverage, because this is the jest perimeter to pick the right configuration for each package. Whatever your test configuration, at the end you will have a coverage-file.json file that the plugin can analyze. Is it clear for you ?

To conclude,(tell me if I am right), you propose to ignore files outside each package for the code-coverage analysis, right?