Add find_files builtin, use it to fix find_and_copy implementation

The macro find-and-copy finds all the files in the given source tree that
match the given filename patten and create <source>:<dest> pair with the
same relative path in the destination tree.

Bug: 193540681
Test: rbcrun build/make/tests/run.rbc
Change-Id: Ic4315ce2fab7a7791ab55dd9eed039205a1c721a
diff --git a/tests/run.rbc b/tests/run.rbc
index 35ae19d..e392e47 100644
--- a/tests/run.rbc
+++ b/tests/run.rbc
@@ -53,7 +53,7 @@
       "PRODUCT_COPY_FILES": [
           "part_from:part_to",
           "device_from:device_to",
-          "device/google/redfin/audio/audio_platform_info_noextcodec_snd.xml:||VENDOR-PATH-PH||/etc/audio_platform_info_noextcodec_snd.xml",
+          "device/google/redfin/audio/audio_platform_info_noextcodec_snd.xml:||VENDOR-PATH-PH||/etc/audio/audio_platform_info_noextcodec_snd.xml",
           "xyz:/etc/xyz",
           "x.xml:/etc/x.xml",
           "y.xml:/etc/y.xml",