Make PyRuntimeConfiguredTargetTest pass with Starlark py_runtime impl.

Various fixes from the rushed initial implementation
 * ctx.attr.files is a list of targets, not a single target
 * interpreter_path defaults to an empty string when not specified, while
   it must be None for PyRuntimeInfo to accept the non-hermetic case.
 * The files arg must be None for the non-hermetic case (an empty depset is
   not accepted by PyRuntimeInfo).
 * Missing the `.py` part when access the fragment to get the default
   Python version.
 * Default `python_version` to `_INTERNAL_SENTINEL` to match existing
   behavior (otherwise it defaults to empty string)

Work towards #15897

PiperOrigin-RevId: 485109694
Change-Id: Icfbbf606acb8074c12e7dba9b3ec39e6ac524bc5

src/main/starlark/builtins_bzl/common/python/py_runtime_rule.bzl

@@ -18,13 +18,17 @@ load(":common/paths.bzl", "paths")
 _PyRuntimeInfo = _builtins.toplevel.PyRuntimeInfo
 
 def _py_runtime_impl(ctx):
-    interpreter_path = ctx.attr.interpreter_path
+    interpreter_path = ctx.attr.interpreter_path or None  # Convert empty string to None
     interpreter = ctx.file.interpreter
     if (interpreter_path and interpreter) or (not interpreter_path and not interpreter):
         fail("exactly one of the 'interpreter' or 'interpreter_path' attributes must be specified")
 
+    runtime_files = depset(transitive = [
+        t[DefaultInfo].files
+        for t in ctx.attr.files
+    ])
+
     hermetic = bool(interpreter)
-    runtime_files = ctx.attr.files[DefaultInfo].files
     if not hermetic:
         if runtime_files:
             fail("if 'interpreter_path' is given then 'files' must be empty")
@@ -62,11 +66,13 @@ def _py_runtime_impl(ctx):
                 "Python runtime mechanism (`--incompatible_use_python_toolchains=false`).",
             )
         else:
-            python_version = ctx.fragments.default_python_version
+            python_version = ctx.fragments.py.default_python_version
 
     return [
         _PyRuntimeInfo(
-            files = runtime_files,
+            interpreter_path = interpreter_path or None,
+            interpreter = interpreter,
+            files = runtime_files if hermetic else None,
             coverage_tool = coverage_tool,
             coverage_files = coverage_files,
             python_version = python_version,
@@ -152,6 +158,7 @@ the `run` and `lcov` subcommands.
 """,
         ),
         "python_version": attr.string(
+            default = "_INTERNAL_SENTINEL",
             values = ["PY2", "PY3", "_INTERNAL_SENTINEL"],
             doc = """
 Whether this runtime is for Python major version 2 or 3. Valid values are `"PY2"`