Closes https://github.com/flutter/flutter/issues/160219.
This hard-coded package configuration is no longer strictly correct
as-of Dart 3.6.0; a pub _workspace_
(https://dart.dev/tools/pub/workspaces) can appear at a higher-level
than a package, and if the package is part of the workspace, tooling is
expected to (automatically) find `.dart_tool/package_config.json` at a
higher-level.
For example, the _engine_ sub-repo uses a
[workspace](9fd5bddc65/engine/src/flutter/pubspec.yaml (L82)),
which means that, for example,
`%ENGINE%/tools/engine_tool/.dart_tool/package_config.json` will _never_
exist (it will be at `%ENGINE%/.dart_tools/package_config.json`.
As currently defined, the test-golden comparator interface will fail
with a package that uses workspaces. By removing the flag (and letting
automatic `--packages` resolution occur), I _believe_ the problem is
automatically resolved (but I'll let CI prove that for us).
0c3359df8c