build: exact per-binary imports — the pre-wired default set is gone

Every binary's build.zig now names precisely the modules its source
imports (derived by scanning each artifact's sources, transitively
through same-directory files), and its zon carries only the domains
those come from — kernel stays implicit (the root shim + link script
live there). build-support's userBinary resolves each name through one
module-to-domain table (module_homes); Domains/domains()/defaultImports
and the raw recipe entry point are deleted. An undeclared @import is a
compile error (verified: injecting @import("xkeyboard-config") into
logger fails with 'no module named ... available within module
program'), and e.g. xkeyboard-config now appears in exactly two
manifests — the two keyboard drivers. Availability never bloated the
emitted binaries (Zig compiles only what a program imports); this makes
the declared interfaces honest. Production and -Dtest-case manifests
byte-identical; all build variants and standalone package builds
green.

system/drivers/usb-xhci-bus/build.zig.zon

@@ -4,11 +4,13 @@
     .fingerprint = 0x46d21373f05f6b20, // Changing this has security and trust implications.
     .minimum_zig_version = "0.16.0",
     .dependencies = .{
+        // build-support supplies the shared recipe; kernel is implicit in
+        // every binary (the root shim + link script live there). The rest
+        // are exactly the homes of this binary's declared imports.
         .@"build-support" = .{ .path = "../../../build-support" },
         .kernel = .{ .path = "../../../library/kernel" },
-        .device = .{ .path = "../../../library/device" },
         .client = .{ .path = "../../../library/client" },
-        .@"xkeyboard-config" = .{ .path = "../../../library/xkeyboard-config" },
+        .device = .{ .path = "../../../library/device" },
         .protocol = .{ .path = "../../../library/protocol" },
     },
     .paths = .{""},