Allow $schema in XSLT-generated JSON schemas

For usnistgov#383, allow $schema so JSON schemas that are
emitted by the XSLT-M4 implementation allow downstream consumers to
optionally bind schema references into a document instances and not
immediately invalidate the schema.

toolchains/xslt-M4/schema-gen/make-json-schema-metamap.xsl

@@ -41,9 +41,16 @@
 
             <string key="type">object</string>
             <map key="definitions">
+                <map key="json-schema-directive">
+                    <string key="title">Schema Directive</string>
+                    <string key="description">A JSON Schema directive to bind a specific schema to its document instance.</string>
+                    <string key="$id">#json-schema-directive</string>
+                    <string key="$ref">#/definitions/URIReferenceDatatype</string>
+                </map>
                 <xsl:apply-templates select="define-assembly | define-field"/>
 
-                <xsl:variable name="all-used-types" select="//@as-type => distinct-values()"/>
+                <xsl:variable name="schema-directive-type" select="'uri-reference'"/>
+                <xsl:variable name="all-used-types" select="(//@as-type, $schema-directive-type) => distinct-values()"/>
                 <xsl:variable name="used-atomic-types" select="$datatype-map[@as-type = $all-used-types]"/>
                 <xsl:variable name="invoked-atomic-types" select="$used-atomic-types/key('datatypes-by-name',string(.),$datatypes)"/>
                 <xsl:variable name="referenced-types" select="$invoked-atomic-types//*:string[@key='$ref']/substring-after(.,'#/definitions/') ! key('datatypes-by-name',string(.),$datatypes)"/>
@@ -75,6 +82,9 @@
 
     <xsl:template match="define-assembly" mode="root-requirement">
         <map key="properties">
+            <map key="$schema">
+                <string key="$ref">#json-schema-directive</string>
+            </map>
             <map key="{root-name}">
                 <xsl:apply-templates select="." mode="make-ref"/>
             </map>