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/w6a/asm.ww

@@ -1,11 +1,10 @@
 // Port of cmd/w6a/asm.c. Encoding subset matches what w6c emits — see
 // cmd/w6a/asm.c for the authoritative list.
 
-package w6a;
+package main;
 
 import os;
 import rt;
-import opcodes;
 import strings;
 
 export fn emitbyte(a: *asm_, b: u8) void = {

selfhost/cmd/w6a/lex.ww

@@ -1,6 +1,6 @@
 // Port of cmd/w6a/lex.c.
 
-package w6a;
+package main;
 
 export fn isidstart(c: i32) bool = {
 	if (c == 95) { return true; };

selfhost/cmd/w6a/main.ww

@@ -7,11 +7,6 @@ package main;
 import os;
 import rt;
 import strings;
-import opcodes;
-import lex;
-import parse;
-import asm;
-import obj;
 
 fn cstreq(a: *u8, lit: str) bool = {
 	let n: u64 = lit.len: u64;

selfhost/cmd/w6a/obj.ww

@@ -11,10 +11,9 @@
 //
 // Symtab indices: 0 = STN_UNDEF, 1.. = our syms. Only GLOBAL symbols.
 
-package w6a;
+package main;
 
 import os;
-import opcodes;
 
 // Local wrappers around os.writeall's tagged return — collapse the
 // (i64 | oserror) back to a boolean / int sentinel for the

selfhost/cmd/w6a/opcodes.ww

@@ -1,6 +1,6 @@
 // Mirrors cmd/w6a/a.h and cmd/w6c/6.out.h.
 
-package w6a;
+package main;
 
 // Registers + operand kinds must stay numerically aligned with the
 // 6.out.h C enum so that ww-cgen output (which reads them via

selfhost/cmd/w6a/parse.ww

@@ -10,12 +10,10 @@
 //   instr := \tMNEM\t[OP1[, OP2]]
 //   OP    := $NUM | REG | NUM(REG) | (REG) | name(SB) | label
 
-package w6a;
+package main;
 
 import os;
 import strings;
-import lex;
-import opcodes;
 
 fn streqlit(p: *u8, n: u64, lit: str) bool = {
 	if (n != lit.len: u64) { return false; };