From the the pub package layout doc:
DON'T import implementation files from another package.
The libraries inside
lib are publicly visible: other packages are free to
import them. But much of a package's code is internal implementation libraries
that should only be imported and used by the package itself. Those go inside a
src. You can create subdirectories in there if
it helps you organize things.
You are free to import libraries that live in
lib/src from within other Dart
code in the same package (like other libraries in
lib, scripts in
and tests) but you should never import from another package's
directory. Those files are not part of the package's public API, and they
might change in ways that could break your code.
// In 'road_runner' import 'package:acme/lib/src/internals.dart;