build: delete module_homes — imports resolve through the declared zon

The module-to-domain table in build-support duplicated what each
domain's build.zig already states with its addModule exports. userBinary
now resolves each named import by searching the packages the binary
declared in its own build.zig.zon (b.available_deps), which also makes
the zon the literal include path: an import can only be satisfied by a
domain the binary claims, and naming a module whose domain is missing
fails the build graph with the domain to declare. build-support is down
to the recipe alone. All build variants green; manifest unchanged.

test/system/services/thread-test/build.zig

@@ -1,6 +1,6 @@
 //! The thread-test test fixture as a binary package (docs/build-packages-plan.md):
 //! this file names the binary and EXACTLY the modules its source imports —
-//! the shared recipe and the module-to-domain map live in build-support.
+//! build-support resolves each name from the domains this zon declares.
 
 const std = @import("std");
 const build_support = @import("build-support");