I plan to extend the prepare_package.dart script to upload the flutter preview device ([design doc](https://docs.google.com/document/d/1AzI-_Uk2v1LA2kKKFJ7gVD4xcakXJ6yVZiS5Ek6RHtg/edit#heading=h.byp03plw7mg9)).

However, given that that script is one large >1k line file, I decided to organize it into smaller libraries in this PR. There should be no behavioral change in this PR, this is a cleanup only. I made the following changes:

1. Created a //dev/bots/prepare_package/ directory to contain helper libraries
2. Moved everything but the `main()` function in //dev/bots/prepare_package.dart into one of 4 helper libraries under the new directory from step 1:
  a. archive_creator.dart which contains the code that creates archive directory locally on disk
  b. archive_publisher.dart which contains the code that uploads the archive to cloud storage
  c. common.dart for shared constants and definitions
  d. process_runner.dart for an abstraction over running sub-processes
3. Changed all definitions to `File` and `Directory` from `dart:io` to use the testable versions from `package:file`. This allowed me to use the `MemoryFileSystem` in the unit tests, rather than creating real temp file system directories.