VM still supports `.packages` files

[lrhn]

The VM still supports package configuration using .packages files when spawning isolates.

The test tests/lib/isolate/scenarios/automatic_resolution_spec/package_resolve_test.dart tests this, using a .packages file in the same directory.

A .packages file is otherwise no longer supported. The language versioning feature was introduced in Dart 2.8, and while it still supported .packages at that point, any package configured through a .packages file would have a default language version of 2.7.
Since Dart 3.0, language version 2.7 is no longer supported, so any package configured by a .packages file must no longer compile.

The VM should just stop supporting .packages files. So should any other tool which is Dart 3.0 compatible.

[lrhn]

I did a quick-and-dirty tear our of the .packages support code in the VM:
https://dart-review.googlesource.com/c/sdk/+/412664

It was mainly to test whether if we have something else that depends on it. (Doesn't seem so, which is good.)

I don't now if there is more code elsewhere, but if you can use it as fix, or as a basis for a fix, feel free to take it.

[a-siva]

The above CL was modified a bit and it has landed, closing issue.