depgen: Add error for rate block name conflicts

This can occur during template expansion if the user is not careful in
ensuring that all generated blocks will have unique names.  The error was
not useful: it failed type validation, which results in an internal error in
the compiler (from the user's perspective) and outputs no useful information
to aid in debugging.

* src/current/include/depgen.xsl
  (preproc:depgen preproc:sym): User-friendly error if more than one rate
    element.
  [rate]: Allow multiple rate elements.

src/current/include/depgen.xsl

@@ -275,9 +275,15 @@
   <xsl:variable name="pkg" as="element( lv:package )"
                 select="root(.)" />
 
-  <xsl:variable name="rate" as="element( lv:rate )"
+  <xsl:variable name="rate" as="element( lv:rate )*"
                 select="$pkg/lv:rate[ @yields=$name ]" />
 
+  <xsl:if test="count( $rate ) gt 1">
+    <xsl:message terminate="yes"
+                 select="'error: rate block name conflict:',
+                         string( $rate/@yields[1] )" />
+  </xsl:if>
+
   <xsl:apply-templates mode="preproc:depgen"
                        select="$rate" />
 </xsl:template>