Change Amp root namespace

[schlessera]

Feature description

The Amp root namespace is already used by the Amp asynchronous PHP library: https://github.com/amphp/amp/blob/master/lib/Loop.php#L3

We should use a different one to not cause conflicts.

@sebastianbenz suggested aligning with the NPM @ampproject namespace, so we'd use AmpProject as the root namespace for the PHP libraries.

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Acceptance criteria

• All namespaced libraries use AmpProject as the root namespace.

Implementation brief

• A simple "Rename" refactor in PHPStorm will allow this change in a reliable manner across the PHP codebase.
• The composer.json file will probably need to be adapted manually.
• Test namespaces should be adapted as well.
• In case we already released code with the Amp root namespace, we can use class aliases to maintain compatibility.

QA testing instructions

Demo

Changelog entry