Add synthetic $package$ file, and update docs (dart-lang#2574)

Fixes dart-lang#1132

The docs have been updated to clarify when to use `$package$` versus `lib/$lib$`, as well as adding a deprecation message to the other placeholder files.

Also updates `build_test`. See CHANGELOG.md notes about the potentially breaking change there, which we are choosing to release as a non-breaking change.

Add synthetic $package$ file, and update docs (dart-lang#2574)

Fixes dart-lang#1132

The docs have been updated to clarify when to use `$package$` versus `lib/$lib$`, as well as adding a deprecation message to the other placeholder files.

Also updates `build_test`. See CHANGELOG.md notes about the potentially breaking change there, which we are choosing to release as a non-breaking change.

Add a warning about DDC deprecation

Closes dart-lang#2564

Add a package version that is only picked up by recent SDK releases that
warns that DDC is no longer supported and directs the user to upgrade to
newer releases of the `build_web_compilers` package.

Fail the build during Builder instantiation to avoid doing work that
will be thrown away and spamming the output with errors for every
module.

fix a bug when changing the root package name (dart-lang#2570)

prep build_test 0.10.10 for release (dart-lang#2555)

Update pubspecs/changelogs and prep for release (dart-lang#2548)

* update build package pubspec and prep for release

* update build_resolvers pubspec/changelog and prep for release

* update build_runner_core pubspec/changelog and prep for release

Update pubspecs/changelogs and prep for release (dart-lang#2548)

* update build package pubspec and prep for release

* update build_resolvers pubspec/changelog and prep for release

* update build_runner_core pubspec/changelog and prep for release

Update pubspecs/changelogs and prep for release (dart-lang#2548)

* update build package pubspec and prep for release

* update build_resolvers pubspec/changelog and prep for release

* update build_runner_core pubspec/changelog and prep for release

add BUILD_DART2JS_VM_ARGS environment variable support (dart-lang#2544)

Prepare to publish compilers packages (dart-lang#2522)

Towards dart-lang#2513

Remove dependency overrides for analyzer and for published packages.