Allow optional dependencies in P2

[nedtwigg]

Right now, all P2 dependencies are parsed as mandatory.

e.g.

<!-- example of filter -->
<required name="org.eclipse.swt.gtk.linux.x86_64" namespace="org.eclipse.equinox.p2.iu" range="[3.122.0.v20221123-2302,3.122.0.v20221123-2302]">
  <filter>(&amp;(osgi.arch=x86_64)(osgi.os=linux)(osgi.ws=gtk)(!(org.eclipse.swt.buildtime=true)))</filter>
</required>
<!-- example of optional -->
<required namespace='osgi.bundle' name='org.eclipse.help.base' range='[4.0.0,5.0.0)' optional='true' greedy='false'/>

We parse these as mandatory requirements. That works out fine for the platform-specific requirements, because the dependency targets themselves also have platform filters, and we have implemented those already.

But for optional dependencies, it means that we generate more warnings in equoList --problems than we need to.

That's not terrible, but we could easily do better. Part of the open problem is specifying when to seek out optional dependencies versus when not to...