Dart 2.18: Replace package_spec_action with package_config_action

[cbracken]

Prior to Dart 2.18, package metadata collected by pub was emitted in a package spec (.packages file). This contained a mapping of package name to resolved URL (typically a file URL) for the transitive closure of dependencies of each package. The existing rules emit this file and pass it to the VM and compilers via the --packages option.

Support for package specs was deprecated in 2.8.0 and dropped in Dart 2.18. The replacement is a JSON-based package config file written to .dart_tool/package_config.json. We'll need to add an equivalent to package_spec_action that generates this new file, and update existing rules to make use of it.

This is blocking the roll to Dart 2.18.