Make references to //third_party/icu relative

This allows the build of ICU to be placed in directories other
than //third_party/icu downstream.

The change does not make a difference in code bases that
continue to use //third_party/icu.

Bug: fuchsia:108544
Change-Id: If702e4294aba6fcd45f8691a9b87cade5b4b33d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/3868531
Reviewed-by: Jungshik Shin <jshin@chromium.org>
Reviewed-by: Frank Tang <ftang@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index b7bb4a6..bc70580 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -3,8 +3,8 @@
 # found in the LICENSE file.
 
 import("//build/config/host_byteorder.gni")
-import("//third_party/icu/config.gni")
-import("//third_party/icu/sources.gni")
+import("config.gni")
+import("sources.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
@@ -412,7 +412,7 @@
 
   if (is_chromeos) {
     copy("icudata_hash") {
-      sources = [ "//third_party/icu/chromeos/icudtl.dat.hash" ]
+      sources = [ "chromeos/icudtl.dat.hash" ]
       outputs = [ "$root_out_dir/icudtl.dat.hash" ]
     }
   }