w6c,ww: mangle an imported package's fn main under separate compilation (M4 E3, #99)

The bare-`main` carve-out (which keeps the link entry's main unmangled)
keyed on `leaf == "main" && imported == 0`. Under the combined path a
dependency's body folds in with imported==1, so only the root's main
stayed bare. Under separate compilation each package is its own unit and
a dependency's body carries a path-mangling module-reset but imported==0
(#57) — so an imported `fn main` matched the carve-out, emitted a bare
`TEXT main`, and collided with the root entry (`w6l: duplicate symbol
main`). The combined path was unaffected, so this only surfaced under sep.

Gate the carve-out with sep_isdep = (wwiout != NULL): the producer emits a
.wwi output only for dependency units, never for the root/link-entry unit
(root stripped, #69), symmetric on both stages. Only the root unit's main
now stays bare; an imported package's main mangles on its import path
(e.g. aa.bb.main). Both stages.

Gate: test/wcc/989_depmain_sep.c (table-driven, dotted + single-component
shapes, both stages; asserts the mangled dep main + a single bare root
main + cs==ww byte-id; combined path stays neutral).

cmd/w6c/gc.h

@@ -54,6 +54,14 @@ struct Cg {
 				 * the dep's real definition (link collision).
 				 * Off on the combined path (every existing
 				 * invocation) so M3 is a pure addition. */
+	int	sep_isdep;	/* #99: this unit is a sep DEPENDENCY, not the
+				 * root/link-entry unit. Set from `wwiout != NULL`
+				 * in main: the producer passes -I (.wwi output)
+				 * to DEP units only — the root's .wwi is stripped
+				 * (#69), so wwiout==NULL <=> root/link-entry unit.
+				 * Gates the bare-`main` carve-out: a dep's `fn
+				 * main` must mangle on its path like any decl;
+				 * only the root entry stays bare. */
 };
 
 /* cgen.c */