The whole idea of having two paths to look for schemas (one local, for
module, and one global, in $(OUTDIR)) is bad anyway, because there is no
way to do it in XSLT without warning if the checked schema does not
exist in the first path. I see two ways out of that:
1. rewrite schema_val.xsl in a decent language
2. split the node reference check in schema_val.xsl into two phases: the
   first phase returns a list of subpaths to check, a helper script
   processes that, finds the real paths and feeds it to the second phase
   XSL, which does the internal check.

Change-Id: Icfe3f9ec0960cfd3423c1223ad8f495d7634b25a