[api_summary] Track pubspec.yaml executables section

[kevmoo]

Currently, api_summary does not check the executables section of pubspec.yaml. For CLI utilities and hybrid packages, entries in executables: define the public CLI boundary. Removing or renaming an entry breaks consumers and CI scripts invoking dart run or dart pub global run, which constitutes a major breaking change.

Example

Changing:

executables:
  api_summary:

to:

executables:
  summary:

or removing it entirely currently produces identical api_summary output.