Extend the syntax to 'import vmod [as name] [from "path"];'

There are a surprising number of subtle corner cases here, and I
think I have handled them all correctly, but caveat emptor...

bin/varnishtest/tests/m00000.vtc

@@ -11,10 +11,12 @@ varnish v1 -vcl+backend {
 	import debug;
 	import vtc;
 	import debug;		// again
+	import debug as dbg;
+	import debug as dbg;	// again
 
 	sub vcl_init {
-		new objx = debug.obj();
-		debug.vsc_new();
+		new objx = dbg.obj();
+		dbg.vsc_new();
 	}
 
 	sub vcl_synth {

bin/varnishtest/tests/m00008.vtc

@@ -12,6 +12,11 @@ varnish v1 -errvcl {Module debug conflicts with other symbol.} {
 	}
 }
 
+varnish v1 -errvcl {Another module already imported as foo} {
+	import debug as foo;
+	import directors as foo;
+}
+
 server s1 {
 	rxreq
 	txresp

lib/libvcc/vcc_compile.h

@@ -144,7 +144,7 @@ struct symbol {
 	int				hirev;
 
 	struct symbol			*parent;
-	const struct symbol		*vmod;
+	const char			*vmod_name;
 
 	sym_wildcard_t			*wildcard;
 	vcc_kind_t			kind;

lib/libvcc/vcc_expr.c

@@ -359,10 +359,10 @@ vcc_priv_arg(struct vcc *tl, const char *p, const struct symbol *sym)
 	struct procprivhead *marklist = NULL;
 
 	AN(sym);
-	AN(sym->vmod);
+	AN(sym->vmod_name);
 
 	if (!strcmp(p, "PRIV_VCL"))
-		return (vcc_mk_expr(VOID, "&vmod_priv_%s", sym->vmod->name));
+		return (vcc_mk_expr(VOID, "&vmod_priv_%s", sym->vmod_name));
 
 	if (!strcmp(p, "PRIV_CALL")) {
 		bprintf(buf, "vmod_priv_%u", tl->unique++);
@@ -383,9 +383,9 @@ vcc_priv_arg(struct vcc *tl, const char *p, const struct symbol *sym)
 	}
 	AN(f);
 	AN(marklist);
-	bprintf(buf, "ARG_priv_%s_%s", f, sym->vmod->name);
+	bprintf(buf, "ARG_priv_%s_%s", f, sym->vmod_name);
 
-	if (vcc_MarkPriv(tl, marklist, sym->vmod->name) == NULL)
+	if (vcc_MarkPriv(tl, marklist, sym->vmod_name) == NULL)
 		VSB_printf(tl->curproc->prologue,
 			   "  struct vmod_priv *%s = "
 			   "VRT_priv_%s(ctx, &VGC_vmod_%s);\n"
@@ -394,7 +394,7 @@ vcc_priv_arg(struct vcc *tl, const char *p, const struct symbol *sym)
 			   "for vmod %s\");\n"
 			   "    return;\n"
 			   "  }\n",
-			   buf, f, sym->vmod->name, buf, f, sym->vmod->name);
+			   buf, f, sym->vmod_name, buf, f, sym->vmod_name);
 	return (vcc_mk_expr(VOID, "%s", buf));
 }
 

lib/libvcc/vmodtool.py

@@ -603,7 +603,7 @@ class ModuleStanza(Stanza):
             write_rst_hdr(fo, "SYNOPSIS", "=")
             fo.write("\n")
             fo.write(".. parsed-literal::\n\n")
-            fo.write('  import %s [from "path"]\n' % self.vcc.modname)
+            fo.write('  import %s [as name] [from "path"]\n' % self.vcc.modname)
             fo.write("  \n")
             for c in self.vcc.contents:
                 c.synopsis(fo, man)