build: phase 2 pilot — pci-bus builds as a package

build-support gains the domains-based userBinary: the default import set
(the library/kernel concern modules, driver/service clients, mmio,
acpi-ids, xkeyboard-config) is assembled from the domain packages, and
the root shim + user link script come from the kernel package directory.
system/drivers/pci-bus is the first binary package: a ~15-line
declarative build.zig naming only its extras (device-manager-protocol,
pci-class); the root build consumes the artifact for the boot image and
the driver also builds standalone from its own directory. Boot-image
file list unchanged.

system/drivers/pci-bus/build.zig.zon

@@ -0,0 +1,15 @@
+.{
+    .name = .pci_bus,
+    .version = "0.0.0",
+    .fingerprint = 0x283fca121f0bb145, // Changing this has security and trust implications.
+    .minimum_zig_version = "0.16.0",
+    .dependencies = .{
+        .@"build-support" = .{ .path = "../../../build-support" },
+        .kernel = .{ .path = "../../../library/kernel" },
+        .device = .{ .path = "../../../library/device" },
+        .client = .{ .path = "../../../library/client" },
+        .@"xkeyboard-config" = .{ .path = "../../../library/xkeyboard-config" },
+        .protocol = .{ .path = "../../../library/protocol" },
+    },
+    .paths = .{""},
+}