Add flet if no dependencies provided for flet publish command

[FeodorFitsner]

Fix #4493

Summary by Sourcery

Bug Fixes:

• Ensure the flet publish command includes flet as a dependency if no other dependencies are provided.

[ndonkoHenri]

• What happens if len(dependencies) > 0 (other packages are specified) but Flet is forgotten/omitted? Should we have that considered too?

• do we need such checks in build too?

[FeodorFitsner]

What happens if len(dependencies) > 0 (other packages are specified) but Flet is forgotten/omitted? Should we have that considered too?

That's the whole fun. In pre-0.25 we were adding flet dependency here and there, but I realized that at some cases it could be indirect dependency, for example someawesome-flet-package could depend on flet and requirements.txt could have only someawesome-flet-package listed. I think we should assume the app has flet as dependency if requirements.txt is not empty 🤨

[ndonkoHenri]

Yeah, makes sense!