PHP Invalid paths in composer.json

[NicklasWallgren]

The composer.json contains invalid paths.

"src/Google/Protobuf/descriptor.php" should be "php/src/Google/Protobuf/descriptor.php"

https://github.com/google/protobuf/blob/master/composer.json

 "autoload": {
    "psr-4": {
      "Google\\Protobuf\\Internal\\": "src/Google/Protobuf/Internal",
      "GPBMetadata\\Google\\Protobuf\\Internal\\": "src/GPBMetadata/Google/Protobuf/Internal"
    },
    "files": [
      "src/Google/Protobuf/descriptor.php"
    ]
  }

[NicklasWallgren]

@TeBoring Could you have a look? :)

[NicklasWallgren]

#2538

[TeBoring]

#2538 merged.