add prebuilt_kernel_modules module

prebuilt_kernel_modules is a build rule for processing and installing
kernel module files. The module files are processed by depmod to build
modules.{load|alias|deps|softdeps} files. Debug symbols are stripped
from the module files and then all the files are installed to
./lib/modules/<kernel_version> directory.

This is the Soong-equivalent of the 'build-image-kernel-modules-dir`
macro and is expected to replace it in the future.

Bug: 178980227
Test: m

Change-Id: I772ac816824921d275869e35b93a083d239a2450
diff --git a/android/packaging.go b/android/packaging.go
index 7a14479..df2533f 100644
--- a/android/packaging.go
+++ b/android/packaging.go
@@ -49,6 +49,11 @@
 	return ""
 }
 
+// Path relative to the root of the package
+func (p *PackagingSpec) RelPathInPackage() string {
+	return p.relPathInPackage
+}
+
 type PackageModule interface {
 	Module
 	packagingBase() *PackagingBase