php: improve support for metapackages

[raboof]

composer has the concept of 'metapackages', that only have dependencies an no 'own' code. An example of a metapackage is https://github.com/symfony/polyfill-php72 - this one doesn't even have dependencies.

I have a situation where beberlei/DoctrineExtensions 1.5.0 has a dependency on doctrine/orm 2.20.8. The derivation for doctrine/orm correctly has:

{"options":{"symlink":false,"versions":{"symfony/polyfill-php72":"v1.31.0"}},"type":"path","url":"/nix/store/1grysa45p9yl659apjr0yjqfm20mdfgy-symfony-polyfill-php72-v1.31.0/lib/vendor/symfony/polyfill-php72"}

However, beberlei/DoctrineExtensions has:

{"options":{"symlink":false,"versions":{"symfony/polyfill-php72":"v1.31.0"}},"type":"path","url":"/nix/store/jc76878ssh0kdh7x5ci1v1ylvn6j9k4c-doctrine-orm-2.20.8/lib/vendor/doctrine/orm/vendor/symfony/polyfill-php72"}

... which does not exist, causing it to fail with:

Running phase: unpackPhase
@nix { "action": "setPhase", "phase": "unpackPhase" }
Running phase: patchPhase
@nix { "action": "setPhase", "phase": "patchPhase" }
Running phase: updateAutotoolsGnuConfigScriptsPhase
@nix { "action": "setPhase", "phase": "updateAutotoolsGnuConfigScriptsPhase" }
Running phase: configurePhase
@nix { "action": "setPhase", "phase": "configurePhase" }
Running phase: buildPhase
@nix { "action": "setPhase", "phase": "buildPhase" }
No composer.lock file present. Updating dependencies to latest instead of installing from lock file. See https://getcomposer.org/install for more information.
Loading composer repositories with package information

In PathRepository.php line 163:
                                                                               
  The `url` supplied for the path (/nix/store/jc76878ssh0kdh7x5ci1v1ylvn6j9k4  
  c-doctrine-orm-2.20.8/lib/vendor/doctrine/orm/vendor/symfony/polyfill-php72  
  ) repository does not exist                                                  
                                                                               

install [--prefer-source] [--prefer-dist] [--prefer-install PREFER-INSTALL] [--dry-run] [--download-only] [--dev] [--no-suggest] [--no-dev] [--no-autoloader] [--no-progress] [--no-install] [--audit] [--audit-format AUDIT-FORMAT] [-v|vv|vvv|--verbose] [-o|--optimize-autoloader] [-a|--classmap-authoritative] [--apcu-autoloader] [--apcu-autoloader-prefix APCU-AUTOLOADER-PREFIX] [--ignore-platform-req IGNORE-PLATFORM-REQ] [--ignore-platform-reqs] [--] [<packages>...]

It's not yet super obvious to me whether we should adapt the build of doctrine/orm so that /nix/store/jc76878ssh0kdh7x5ci1v1ylvn6j9k4c-doctrine-orm-2.20.8/lib/vendor/doctrine/orm/vendor/symfony/polyfill-php72 does exist, or the build of beberlei/DoctrineExtensions so that it links directly to the polyfill instead.

Any pointers welcome!