map: Provide useful warnings when missing import/@path

This is all really confusing because this doesn't use the same import
specification as packages; maps got stuck in a partial transition.  So,
let's provide some helpful errors rather than silently failing.

* src/current/compiler/map.xsl (preproc:symtable)[lvm:import]:
    Error if missing `@path'.  Provide more information if `@package' was
    provided to help clarify.

src/current/compiler/map.xsl

@@ -869,13 +869,23 @@
 
 
 <!-- import symbols -->
-<template match="lvm:import[ @path ]" mode="preproc:symtable" priority="5">
+<template match="lvm:import" mode="preproc:symtable" priority="5">
   <!-- original root passed to sym-discover -->
   <param name="orig-root" />
 
   <variable name="src" as="xs:string"
             select="@path" />
 
+  <if test="not( @path )">
+    <message terminate="yes"
+             select="concat(
+                       'fatal: import missing @path',
+                       if ( @package ) then
+                           ' (use instead of @package)'
+                         else
+                           () )" />
+  </if>
+
   <!-- perform symbol import -->
   <call-template name="preproc:symimport">
     <with-param name="orig-root" select="$orig-root" />