Make the bazel sandwich ignore targets that don't exist

So that things can be allowlisted that aren't in all branches, or
that aren't present in tests.

Then remove the example partition from the soong integration tests
so that we don't need to pull all of the tools needed to build
partitions into that test.

Bug: 265127181
Fixes: 294220392
Test: ./build/soong/tests/run_integration_tests.sh and m bazel_sandwich
Change-Id: I66fb1ad1c2eee815bfb9700595a26e74e54c6214

android/bazel_handler.go

@@ -22,6 +22,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"sort"
 	"strings"
@@ -1054,9 +1055,23 @@ var (
 
 func GetBazelSandwichCqueryRequests(config Config) ([]cqueryKey, error) {
 	result := make([]cqueryKey, 0, len(allowlists.BazelSandwichTargets))
+	labelRegex := regexp.MustCompile("^@?//([a-zA-Z0-9/_-]+):[a-zA-Z0-9_-]+$")
 	// Note that bazel "targets" are different from soong "targets", the bazel targets are
 	// synonymous with soong modules, and soong targets are a configuration a module is built in.
 	for _, target := range allowlists.BazelSandwichTargets {
+		match := labelRegex.FindStringSubmatch(target.Label)
+		if match == nil {
+			return nil, fmt.Errorf("invalid label, must match `^@?//([a-zA-Z0-9/_-]+):[a-zA-Z0-9_-]+$`: %s", target.Label)
+		}
+		if _, err := os.Stat(absolutePath(match[1])); err != nil {
+			if os.IsNotExist(err) {
+				// Ignore bazel sandwich targets that don't exist.
+				continue
+			} else {
+				return nil, err
+			}
+		}
+
 		var soongTarget Target
 		if target.Host {
 			soongTarget = config.BuildOSTarget