add package:_macros (SDK vendored) and package:macros (pub published)

add sdk_packages.yaml file (describes SDK vendored package locations)

delete old macro code in _fe_analyzer_shared, move tests/benchmarks

adds a top level `pkg` directory to the Dart SDK, which is where vendored packages live

BUG: #54976
Change-Id: Ib3503a27fb5644fa8a39ab5a3e5b568df330cfd6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/359040
Auto-Submit: Jake Macdonald <jakemac@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Jonas Termansen <sortie@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>
Reviewed-by: Devon Carew <devoncarew@google.com>

Author: scheglov

OWNERS

@@ -15,7 +15,7 @@ per-file DEPS=file:/tools/OWNERS_ENG
 per-file CHANGELOG.md,AUTHORS,WATCHLISTS,.gitattributes,.gitconfig,.gitignore,sdk.code-workspace=*
 
 # Product documentation
-per-file CONTRIBUTING.md,LICENSE,PATENT_GRANT,README.*,SECURITY.md=file:/tools/OWNERS_PRODUCT
+per-file CONTRIBUTING.md,LICENSE,PATENT_GRANT,README.*,SECURITY.md,sdk_packages.yaml=file:/tools/OWNERS_PRODUCT
 
 # Top level build files
 per-file .clang-format,.gn,BUILD.gn,sdk_args.gni=file:/tools/OWNERS_BUILD