[coverage] Pub workspace support

[dcharkes]

We should start supporting pub workspaces in package coverage.

Intended workflow: dart pub global run coverage:test_with_coverage in a workspace directory, it should produce a coverage file spanning all sources of all packages in that workspace. Lines covered by the tests of another package should also count as coverage.

@liamappelbe Things that need to change:

• The file filtering
• Multiple different coverage runs for combining