Skip keys that are not meant to be references

[kohlerdominik]

Hi @NickSdot

you pinged me in cebe#156 (comment). I know, it has been a while since then, but the urge to dive into the topic again wasn't that big, to be honest.

Once again I'm now at the good old problem that we can't update the package, so I'll give it another try. I tried your suggested performance fix, and it works 🎉. Well done!

I found this little bug, though, that resolveRelativeUri() is called also on values that are not meant to be URIs. I guess this was unintentionally introduced when trying to DRY the code by combining

$oContextUri = $oContext->getUri();
$resolvedUri = $context->resolveRelativeUri($value);

before

if ($key === '$ref') { ... }
if ($key === 'externalValue') { ... }

So this PR fixes that

[NickSdot]

Good catch, thank you! 🎉