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).

selfhost/cmd/w6c/main.ww

@@ -195,6 +195,10 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	let cg: cgen;
 	cgeninit(&cg);
 	cg.sepmode = sepmode;
+	// #99: wwiout != nil <=> this is a sep DEP unit (the producer passes
+	// -I to deps only; the root's .wwi is stripped per #69). Gates the
+	// bare-`main` carve-out so only the root/link-entry main stays bare.
+	if (wwiout != nil) { cg.sepisdep = 1i32; };
 	cgfile(&cg, f);
 	return 0;
 };