[in_app_purchase] Move in_app_purchase into sub-folder.

[mvanbeusekom]

This moves the in_app_purchase app facing package into a new sub-folder (in_app_purchage/in_app_purchase)
to prepare for the migration to federated architecture.

Note: since this only changes the directory layout of the project but doesn't change anything in the plugin itself I decided not to update the version number in the pubspec.yaml and/or CHANGELOG.md.

Implements part of issue flutter/flutter#78525

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
  • I believe this is not necessary since this change doesn't require a release to pub.dev
• I updated CHANGELOG.md to add a description of the change.
  • I believe this is not necessary since this change doesn't require a release to pub.dev
• I updated/added relevant documentation (doc comments with ///).
  • This change doesn't require any documentation to be updated.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
  • Since the update only changes the projects directory structure and no new code was added there is no need to write additional tests. Of course I made sure all existing tests are passing.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[BeMacized]

LGTM! Waiting to see if the tree goes green

[cyanglaz]

@jmagman This PR moves the plugin code to a sub folder but the iOS build failed when import in_app_purchase module.
Module 'in_app_purchase' not found

Did you know what might cause this?

[cyanglaz]

Thanks to @jmagman finding the issue. I have created a PR implementing the fix Jenn suggested in a separate PR, it should be fixed now. #3744
I'm closing this PR in favor of the above one.

[jmagman]

@jmagman This PR moves the plugin code to a sub folder but the iOS build failed when import in_app_purchase module.
Module 'in_app_purchase' not found

Did you know what might cause this?

It was a redux of flutter/flutter#67175