Change symlink_forest to use relative symlinks.

Also add script to remove the Bazel output base.
This will assist with supporting movable checkouts alongside
mixed builds.

Bug: 259191764
Test: m && (move topic and prepare_moved_top.sh) && m
Test: m && prepare_moved_top.sh && m
Test: build/soong/tests/relative_symlinks_test.sh

Change-Id: I0f53da8d99f752fad496cf3ac61b01f001b7296d

scripts/prepare-moved-top.sh

@@ -0,0 +1,41 @@
+#!/bin/bash -eu
+
+###############
+# Removes the Bazel output base and ninja file.
+# This is intended to solve an issue when a build top is moved.
+# Starlark symlinks are absolute and a moved build top will have many
+# dangling symlinks and fail to function as intended.
+# If the bazel output base is removed WITHOUT the top moving,
+# then any subsequent builds will fail as soong_build will not rerun.
+# Removing the ninja file will force a re-execution.
+#
+# You MUST lunch again after moving your build top, before running this.
+###############
+
+if [[ ! -v ANDROID_BUILD_TOP ]]; then
+    echo "ANDROID_BUILD_TOP not found in environment. Please run lunch before running this script"
+    exit 1
+fi
+
+if [[ ! -v OUT_DIR ]]; then
+    out_dir="$ANDROID_BUILD_TOP/out"
+else
+    out_dir="$ANDROID_BUILD_TOP/$OUT_DIR"
+fi
+
+output_base=$out_dir/bazel/output/
+ninja_file=$out_dir/soong/build*ninja
+
+if [[ ! -d $output_base ]]; then
+    echo "The specified output directory doesn't exist."
+    echo "Have you rerun lunch since moving directories?"
+    exit 1
+fi
+
+read -p "Are you sure you want to remove $output_base and the ninja file $ninja_file? Y/N " -n 1 -r
+echo
+if [[ $REPLY =~ ^[Yy]$ ]]
+then
+   rm -rf $output_base
+   rm $ninja_file
+fi