Reland "Migrate to use pub workspace"

This is a reland of commit b9b77058a9

Original change's description:
> Migrate to use pub workspace
>
> Use `pub get` to generate `.dart_tool/package_config.json` on gclient sync.
>
> All pkg/ (and a few third_party) packages that are developed inside the sdk repo are included in the workspace from the root `pubspec.yaml`.
>
> All dependencies that are pulled in via DEPS are added as path dependencies via `dependency_overrides` in the root `pubspec.yaml`.
>
> Bug: https://github.com/dart-lang/sdk/issues/56220
> Change-Id: I38c12b608c68da54c57821116cf9aa6696936746
> Tested: relies on CQ of existing tests. Should have no effect on functionality
> CoreLibraryReviewExempt: only core library change is adding a `// ignore:` comment. Should have no influence on functionality
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397164
> Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
> Reviewed-by: Alexander Thomas <athom@google.com>

Bug: https://github.com/dart-lang/sdk/issues/56220
Change-Id: I29afabade2d2447dea05121cb87ff50bb21a4b76
Cq-Include-Trybots: luci.dart.try:flutter-linux-try,flutter-web-try
Tested: relies on CQ of existing tests. Should have no effect on functionality
CoreLibraryReviewExempt: only core library change is adding a `//
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/415561
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

pkg/mmap/pubspec.yaml

@@ -6,6 +6,8 @@ publish_to: none
 environment:
   sdk: ^3.5.0
 
+resolution: workspace
+
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dependencies:
   ffi: any