Add a showcommands function to envsetup.

This is similar to the old `m showcommands`, but operates on specific
output paths instead and does not require a build aside from the
initial invocation of soong to generate the ninja files. The
`--regenerate` option is accepted that causes the ninja file to be
regenerated first, but this is not the default because it is slow.
Just note that without this flag results may be stale.

It unfortunately doesn't work for everything in out. For example,
`$OUT/system/lib/libc.so` won't work because it is a symlink that
isn't in the build graph, but `$OUT/system/lib/bootstrap/libc.so`
works.

Test: showcommands out/target/product/walleye/system/lib/libz.so
Bug: None
Change-Id: I4cfded3db994044870fc71ae4bcf9a69224b1e15

envsetup.sh

@@ -1613,6 +1613,41 @@ function source_vendorsetup() {
     done
 }
 
+function showcommands() {
+    local T=$(gettop)
+    if [[ -z "$TARGET_PRODUCT" ]]; then
+        >&2 echo "TARGET_PRODUCT not set. Run lunch."
+        return
+    fi
+    case $(uname -s) in
+        Darwin)
+            PREBUILT_NAME=darwin-x86
+            ;;
+        Linux)
+            PREBUILT_NAME=linux-x86
+            ;;
+        *)
+            >&2 echo Unknown host $(uname -s)
+            return
+            ;;
+    esac
+    if [[ -z "$OUT_DIR" ]]; then
+      if [[ -z "$OUT_DIR_COMMON_BASE" ]]; then
+        OUT_DIR=out
+      else
+        OUT_DIR=${OUT_DIR_COMMON_BASE}/${PWD##*/}
+      fi
+    fi
+    if [[ "$1" == "--regenerate" ]]; then
+      shift 1
+      NINJA_ARGS="-t commands $@" m
+    else
+      (cd $T && prebuilts/build-tools/$PREBUILT_NAME/bin/ninja \
+          -f $OUT_DIR/combined-${TARGET_PRODUCT}.ninja \
+          -t commands "$@")
+    fi
+}
+
 validate_current_shell
 source_vendorsetup
 addcompletions