php: add translator composer-lock

[tinybeachthor]

Towards #240

Seems like composer.lock does not write hashes for dependency archives,
for a pure translator we will need to use git sources only, therefore building everything from source.

TODO:

• resolve requires into pinned versions
• php version
• php extensions
• dev dependencies

[DavHau]

It's very nice to have support for php now.
Possible future work:

• add example for php to /examples
• add subsystem entry for php to the docs
• add impure translator for projects which do not contain a composer lock file (Does that use case make sense?)

[tinybeachthor]

@DavHau

impure translator does make a lot of sense, a lot of php projects don't commit composer.lock (some to get auto-updating-like behaviour, some just don't).
+
Docs entry on php subsystems (translators only so far).
=
#262

Next up builder + examples