feat(tool/cmd/migrate): add support to php

[zhumin8]

Add initial support for php, populate language and sources in the produced librarian.yaml file.

For #6723

[zhumin8]

running migrate now produces starter librarian.yaml:

# Copyright 2026 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
language: php
sources:
  googleapis:
    commit: 71573e67671246bdd4239c0cf42c5c1e03e89f29
    sha256: 4e82da35664ca4d17f44a86d69a2f4a0b76b44277b4595d75e9d406630211bab

[gemini-code-assist]

Code Review

This pull request adds support for PHP migration to the migration tool, introducing the runPHPMigration function and its corresponding test suite. The feedback suggests improving the test implementation by restoring the overridden global fetchSource function using t.Cleanup to prevent side effects in other tests, and using t.Chdir to handle working directory dependencies more cleanly.