selfhost: migrate tool sources to directory packages

Build w6a and w6l from package-main directories and expose the wcc backend through a narrow package API so w6c and wwdump no longer import implementation files. Retarget the remaining load-bearing fixtures and example sources to directory packages; retain the one intentional flat compiler collision as an explicitly composed raw unit.

selfhost/cmd/wcc/cgen.ww

@@ -11,11 +11,6 @@ import memio;
 // Split files. Bundler pulls these in transitively so consumers only
 // need `use cgen;`. Order matters for the flat-bundle concat — utils
 // first so cgenexpr/stmt/decl can reference helpers defined here.
-import cgenutil;
-import cgenexpr;
-import cgenstmt;
-import cgendecl;
-
 // Only direct nkind.N_TNAME aliases are mapped; `type p = struct {...}`
 // is handled by collectstructs.
 
@@ -1419,7 +1414,7 @@ fn preinternstrarray(c: *cgen, au: *syntax.tinfo, r: *syntax.node) void = {
 // emitdatasection emits the DATA row in the same .s file. Running
 // emitletdataw after emitdatasection would flip the (DATA strlits,
 // DATAW lets) section order and break byte-identity.
-export fn letpreintern(c: *cgen, file: *syntax.node) void = {
+fn letpreintern(c: *cgen, file: *syntax.node) void = {
 	if (file == nil) { return; };
 	// #49: strlit labels allocated here (static-data initialisers) take
 	// the OWNING decl's module prefix, not the stale last-fn curmod.
@@ -4033,7 +4028,7 @@ fn argregname(i: i32) str = {
 // fargregname — XMM scalar-float arg registers (SysV: X0..X7).
 // Parallel to argregname / sysv_argregs; float args advance their
 // own counter so int and float arg slots don't conflict.
-export fn fargregname(i: i32) str = {
+fn fargregname(i: i32) str = {
 	if (i == 0) { return "X0"; };
 	if (i == 1) { return "X1"; };
 	if (i == 2) { return "X2"; };