Added GetRunfilesFile() to be able to reliably determine prelude location under various invocation scenarios

The logic is using bazel Runfiles API per Jon's suggestion.

One of the scenarios used by the fuzzing framework is to place the binary and its runfiles into a folder, resulting in a directory layout like this:

[temp dir]
fuzzer
fuzzer.runfiles/

Tested:
bazel test -c opt explorer/fuzzing:explorer_fuzzer
bazel build -c opt explorer/fuzzing:explorer_fuzzer + run the binary from various locations

explorer/fuzzing/fuzzer_util_test.cpp

@@ -36,6 +36,19 @@ TEST(FuzzerUtilTest, RunFuzzerOnCorpus) {
   EXPECT_GT(file_count, 0);
 }
 
+TEST(FuzzerUtilTest, GetRunfilesFile) {
+  EXPECT_THAT(*Internal::GetRunfilesFile(
+                  "carbon/explorer/fuzzing/fuzzer_corpus/empty.textproto"),
+              testing::EndsWith("/explorer/fuzzing/fuzzer_corpus/"
+                                "empty.textproto"));
+  EXPECT_THAT(Internal::GetRunfilesFile(
+                  "explorer/fuzzing/fuzzer_corpus/empty.textproto")
+                  .error()
+                  .message(),
+              testing::EndsWith("fuzzer_util_test.runfiles/explorer/fuzzing/"
+                                "fuzzer_corpus/empty.textproto doesn't exist"));
+}
+
 }  // namespace
 }  // namespace Carbon::Testing