Create app-level metadata for communication between the build system and debugger

[annagrin]

This issue tracks work related to generation and use of app-level metadata in several repos (to be created and added here later)

• SDK:
  • define app-level metadata format (documentation and types) [dartdevc] Generate dart_sdk debug module metadata sdk#45477
• builds: generate app metadata, merged app-level metadata (for multi-ddc scenario)
  • generate metadata for SDK libraries
  • build_web_compilers:
  • blaze
  • flutter tools
• dwds: use app metadata
  • read app level metadata to
    • find locations of individual metadata files
    • set app-level properties, such as root lib, null safety, sdk module and libraries, and more
• builds: remove generating merged metadata
  • build_web_compilers:
  • blaze
  • flutter tools

Needed for:
#1226
dart-lang/sdk#44760
#1030
Better implementation of: #1457 (see discussion in #1472)