php: impure translator composer-json + fix composer-lock

[tinybeachthor]

Towards #240

Found out a couple of issues with composer-lock translator (mostly semver resolution).

• fix php semver resolution
• handle composer dependency replace
• handle composer dependency provide
• impure translator composer-json
• fix dev dependencies resolution
• add php subsystems docs section

[DavHau]

• php example (can we do this without a builder?)

Actually we need a builder first, in order for the example to work.

[tinybeachthor]

PHP packages have their own name but can also replace other packages or provide implementations - both of which can define their own versions.

This PR fixes the composer-lock translator to take replace and provide into account + some issues with the semver parsing (composer semver is a bit relaxed compared to the standard: ~ is treated almost like ^).