WIP: Support XML DTD

[isimluk]

In order to support the latest usnistgov/metaschema, we need to support XML DTD.

usnistgov/metaschema started including XML DTD tags, we need to support these.

[xee5ch]

It seems to support the latest Metaschema, re the linked issue in usnistgov/OSCAL#1665, support for XML DTDs is no longer required for the OSCAL Metaschema models, should we close this @isimluk?

[isimluk]

That's a great news @xee5ch. Thank You!

[isimluk]

FYI: I just found that OSCAL is still depending on DTD in the oscal_control-common_metaschema.xml

https://github.com/usnistgov/OSCAL/blob/4f02dac6f698efda387cc5f55bc99581eaf494b6/src/metaschema/oscal_control-common_metaschema.xml#L2-L8

[mdk-arc]

looks like they are exporting things as "_RESOLVED" to get around this, Using this zip https://github.com/usnistgov/OSCAL/releases/download/v1.1.2/oscal-1.1.2.zip and the "metaschema" folder i was able to get to here:

 Processing  oscal_assessment-common_metaschema_RESOLVED.xml
  Processing imported href: oscal_control-common_metaschema_RESOLVED.xml
  Processing imported href: oscal_metadata_metaschema_RESOLVED.xml
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x50 pc=0x523e41]

goroutine 1 [running]:
github.com/gocomply/metaschema/metaschema/parser.(*Metaschema).linkDefinitions(0xc0002d1600)
	/home/arclight/prj/metaschema/metaschema/parser/metaschema_compile.go:75 +0xc1