chore(build): execute `pub get` only if a pubspec.yaml changed and run `dart analyzer` on all dart files

`pub get` is now only executed when the `pubspec.yaml` in the `modules`
folder is different than the `pubspec.yaml` in the `build/dart` folder.

Generates the file `build/dart/_analyzer.dart` that imports all modules
to run `dart analyzer` against all of them. The build will fail whenever
there are errors, warnings or hints in `dart analyzer`.

Changes the sources so that `dart analyzer` does not report any
error, warning or hint.

Closes #40