Update runfiles lib (#982)

* runfiles: Find runfiles in directories that are themselves runfiles

Cherry-picks a fix added to the Bazel version of the runfiles library in
bazelbuild/bazel@486d153

* runfiles: Add tests from Bazel

python/runfiles/runfiles.py

@@ -200,7 +200,22 @@ def __init__(self, path):
 
     def RlocationChecked(self, path):
         # type: (str) -> Optional[str]
-        return self._runfiles.get(path)
+        """Returns the runtime path of a runfile."""
+        exact_match = self._runfiles.get(path)
+        if exact_match:
+            return exact_match
+        # If path references a runfile that lies under a directory that
+        # itself is a runfile, then only the directory is listed in the
+        # manifest. Look up all prefixes of path in the manifest and append
+        # the relative path from the prefix to the looked up path.
+        prefix_end = len(path)
+        while True:
+            prefix_end = path.rfind("/", 0, prefix_end - 1)
+            if prefix_end == -1:
+                return None
+            prefix_match = self._runfiles.get(path[0:prefix_end])
+            if prefix_match:
+                return prefix_match + "/" + path[prefix_end + 1 :]
 
     @staticmethod
     def _LoadRunfiles(path):

tests/runfiles/BUILD.bazel

@@ -0,0 +1,7 @@
+load("@rules_python//python:defs.bzl", "py_test")
+
+py_test(
+    name = "runfiles_test",
+    srcs = ["runfiles_test.py"],
+    deps = ["//python/runfiles"],
+)