imported-relation relations for usnistgov#1023.

This adds the new `link/@rel` value of `imported-from` in the following
locations in the `system-security-plan`:

- `system-implementation/component/link`
- `control-implementation/implemented-requirements/link`

src/metaschema/oscal_ssp_metaschema.xml

@@ -6,6 +6,7 @@
 <!-- OSCAL Implementation Layer: System Security Plan (SSP) METASCHEMA -->
 <!-- validate with XSD and Schematron (linked) -->
 <!DOCTYPE METASCHEMA [
+  <!ENTITY allowed-values-component_component_link-rel SYSTEM "./shared-constraints/allowed-values-component_component_link-rel.ent">
   <!ENTITY allowed-values-responsible-roles-system SYSTEM "./shared-constraints/allowed-values-responsible-roles-system.ent">
   <!ENTITY allowed-values-responsible-roles-operations SYSTEM "./shared-constraints/allowed-values-responsible-roles-operations.ent">
   <!ENTITY allowed-values-responsible-roles-component-production SYSTEM "./shared-constraints/allowed-values-responsible-roles-component-production.ent">
@@ -720,6 +721,10 @@
       <field ref="remarks" in-xml="WITH_WRAPPER"/>
     </model>
     <constraint>
+      <allowed-values target="link/@rel" allow-other="yes">
+        &allowed-values-component_component_link-rel;
+        <enum value="imported-from">Identifies the UUID or URI the <code>component-definition</code> that originally defined the <code>component</code>.</enum>
+      </allowed-values>
       <allowed-values target="prop/@name" allow-other="yes">
         <enum value="control-origination">Identifies the source of the implemented control.</enum>
       </allowed-values>