Relocate well known packages into alternative php namespaces to resolve conflict with google/protobuf

[kafkiansky]

Thesis is an alternative runtime, so its generated code must be able to live in the same project as the code generated by google/protobuf. That was impossible as long as both generate Google\Protobuf\Any: the class names collide, which is why thesis/protobuf-known-types had to declare conflict: google/protobuf, and that conflict, in turn, made it impossible to use packages tied to google/protobuf (open-telemetry/*, for instance).

This PR teaches the plugin to relocate well known packages into namespaces of its own.

The relocation table is map.json in the repository root. It is bundled into the phar (box.json) and shipped with the composer package, so it applies no matter how the plugin is installed.

{
    "rules": [
        {"from": "google.protobuf.*", "to": "thesis.google.protobuf.*"},
        {"from": "google.rpc.Status", "to": "thesis.google.rpc.Status"}
    ]
}

Rules are written in terms of protobuf names, not PHP ones — the PHP namespace is derived from the rewritten name exactly the same way it is derived from a package.