Make sure that extensions are bundled with the package they claim to be. (#9981)

* Make sure that extensions are bundled with the package they claim to be.

With this change, extensions are enabled if and only if the package name
on disk matches the package name in extension/devtools/config.yaml.

* Check string type in if expression

* Normalize package root

* Fix typo

* Revert missing line in CHANGELOG.md

* Fix presubmits

* Use the package name to verify extension permissions and deduplication

* Use non-nullable parameters

Make extensionPackage a required non-nullable parameter in extensionEnabledState
Make packageName non-nullable parameter in TestPackageWithExtension

* fix PR link

* Add pubspec verification to CHANGELOG

* Fix doc comment in lookupExtensionEnabledState and setExtensionEnabledState

* Format

* Move packageName to correct section in constructor