build: phase 2 wave B — drivers build as packages

ps2-bus (bus + keyboard + mouse artifacts from one package),
usb-xhci-bus, usb-hid (keyboard + mouse), usb-storage, and virtio-gpu
convert on the pci-bus template. Their unit tests — PS/2 decode, HID
boot reports, Bulk-Only Transport/SCSI encodings, and the virtio-gpu
size checks the plan flagged as a wave carry-along — move into their
packages; the root aggregate delegates. Boot-image file list
unchanged.

build.zig.zon

@@ -53,6 +53,11 @@
         .logger = .{ .path = "system/services/logger" },
         .acpi = .{ .path = "system/services/acpi" },
         .fdt = .{ .path = "system/services/fdt" },
+        .@"ps2-bus" = .{ .path = "system/drivers/ps2-bus" },
+        .@"usb-xhci-bus" = .{ .path = "system/drivers/usb-xhci-bus" },
+        .@"usb-hid" = .{ .path = "system/drivers/usb-hid" },
+        .@"usb-storage" = .{ .path = "system/drivers/usb-storage" },
+        .@"virtio-gpu" = .{ .path = "system/drivers/virtio-gpu" },
         // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
         //.example = .{
         //    // When updating this field to a new URL, be sure to delete the corresponding