…of running `dart test --version`
We currently spawn `dart run test:test --version` for each package in the workspace to get the test capabilities during test discovery (to know if we can add profiles like Coverage to a given suite). This can be expensive (in terms of CPU and RAM) if the project is large.
For example opening the Dart SDK root on my machine and then clicking the Test Explorer icon results in all RAM being consumed. We already filter out packages in the same Pub Workspace, but there are lots of non-Workspace packages in there.
With this change, we just read the version from the `.dart_tool/package_graph.json` file instead which is significantly faster and uses little resources.
This also resulted in quite a bit of async code becoming sync.
Fixes #5936