[webview_flutter] Move webview_flutter to webview_flutter/webview_flutter

[mvanbeusekom]

Moves the webview_flutter plugin to webview_flutter/webview_flutter to prepare it to support the federated plugin architecture. This PR solves the task "Move the webview_flutter into its own subdirectory ("webview_flutter/webview_flutter");" listed in issue flutter/flutter#86286.

The PR was created using the following steps (according to the "How To Write a Flutter Web Plugin: Part 2" article):

git mv webview_flutter webview_flutter_tmp
mkdir webview_flutter
git mv webview_flutter_tmp webview_flutter/webview_flutter
git commit -m "Move webview_flutter to webview_flutter/webview_flutter"

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 relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• 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.
  • Not necessary to publish a new version of the plugin as this simply restructures the plugin's directory structure to prepare for federated architecture. The plugin itself doesn't change at all.
• I updated CHANGELOG.md to add a description of the change.
  • Not necessary to publish a new version of the plugin as this simply restructures the plugin's directory structure to prepare for federated architecture. The plugin itself doesn't change at all.
• I updated/added relevant documentation (doc comments with ///).
  • No changes here since the plugin didn't change.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
  • No changes here since the plugin didn't change.
• All existing and new tests are passing.

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