Support xref links into included files

[astubbs]

It will check external files where the file is referenced in the xref, but can't seem to check a xref if the reference is from an include else where in the document.

e.g. include::x.adoc[]

[gaurav-nelson]

Thanks @astubbs for bringing this up.
I recently came across this issue and have been working to fix it. The logic here would be to check all internal xrefs in include files (if any) before reporting it as failed.

[gaurav-nelson]

@astubbs

Also can you share the files/repo where it failed, if its opensource? I'll include those when I test.