PR provides a new option to the `test` command to include coverage info of specified packages.  
It helps collecting coverage info in test setups where test code lives in separate packages or for multi-package projects.
At present, only current package is included to the final report.

Usage:

Consider an app with two packages: `app`, `common`.
Some of the tests in `app` use (indirectly) code that is located in `common`. When running with `--coverage` flag, that code is not included in the coverage report by default. To include `common` package in report, we can run:

```sh
flutter test --coverage --coverage-package app --coverage-package common
```

Note that `--coverage-package` accepts regular expression. 

Fixes https://github.com/flutter/flutter/issues/79661
Fixes https://github.com/flutter/flutter/issues/101486
Fixes https://github.com/flutter/flutter/issues/93619