Explicit dependencies on collection packages to coexist with Freezed 2.0

[riscait]

Explicitly added dependent collection packages to the dependencies.

An explicit dependency is necessary because collection packages are not exported in freezed 2.0.0.

For example, using device_preview in a project using freezed 2.0.0 will result in an error that the groupBy method is not defined.

---

How to use freezed 2.0.0 and device_preview together in a project before this PR is merged

dependency_overrides:
  freezed_annotation: ^2.0.0
  device_preview:
    git:
      url: https://github.com/riscait/flutter_device_preview.git
      path: device_preview
      ref: explicitly-dependent-on-collection

[riscait]

This PR does not fully support Freezed 2.0, but it is a temporary fix.
Related Issue: #180