Fix plugin subclassing in Log4j Docgen

[vy]

This change fixes #117, which misses loggers > logger entry.

Why is the entry missing?

log4j-core-plugins.xml contains two entries for LoggerConfig:

1. <abstractType className="org.apache.logging.log4j.core.config.LoggerConfig">
2. <plugin name="logger" className="org.apache.logging.log4j.core.config.LoggerConfig">

TypeLookup encounters the <abstractType first and consequently discards the <plugin, hence logger doesn't show up in the LoggerConfig XSD group.

How shall we solve the problem?

If an FQCN is a plugin and is also subclassed by other plugins:

1. DescriptorGenerator should not generate an AbstractType, but only a PluginType.
2. TypeLookup should promote an AbstractType to a PluginType if the associated FQCN is later on found to be a plugin. (This can still be the case even after the DescriptorGenerator fix mentioned above. That is, the plugin and the subclassed plugin can be provided in different descriptor files.)

[ppkarwasz]

Looks good to me.

I think that in the long run we could remove the ArtifactSourcedType class and move its additional fields to Type. These fields would have a value of null by default, which would mean that they are equal to the same fields in the PluginSet that contains them, but would be explicitly filled up by TypeLookup.