Change code for handling relative short_paths to fix bug #225. (#231)

Change code for handling relative short_paths.

Fixes #225.

experimental/python/wheel.bzl

diff --git a/experimental/python/wheel.bzl b/experimental/python/wheel.bzl
index 1c43409..a9d35d9 100644
--- a/experimental/python/wheel.bzl
+++ b/experimental/python/wheel.bzl
@@ -15,17 +15,18 @@
 """Rules for building wheels."""
 
 def _path_inside_wheel(input_file):
-    # input_file.short_path is relative ("../${repository_root}/foobar")
-    # so it can't be a valid path within a zip file. Thus strip out the root
-    # manually instead of using short_path here.
-    root = input_file.root.path
-    if root != "":
-        # TODO: '/' is wrong on windows, but the path separator is not available in skylark.
-        # Fix this once ctx.configuration has directory separator information.
-        root += "/"
-    if not input_file.path.startswith(root):
-        fail("input_file.path '%s' does not start with expected root '%s'" % (input_file.path, root))
-    return input_file.path[len(root):]
+    # input_file.short_path is sometimes relative ("../${repository_root}/foobar")
+    # which is not a valid path within a zip file. Fix that.
+    short_path = input_file.short_path
+    if short_path.startswith('..') and len(short_path) >= 3:
+        # Path separator. '/' on linux.
+        separator = short_path[2]
+        # Consume '../' part.
+        short_path = short_path[3:]
+        # Find position of next '/' and consume everything up to that character.
+        pos = short_path.find(separator)
+        short_path = short_path[pos+1:]
+    return short_path
 
 def _input_file_to_arg(input_file):
     """Converts a File object to string for --input_file argument to wheelmaker"""

experimental/rules_python/wheelmaker.py

diff --git a/experimental/rules_python/wheelmaker.py b/experimental/rules_python/wheelmaker.py
index ca3279f..6be5d38 100644
--- a/experimental/rules_python/wheelmaker.py
+++ b/experimental/rules_python/wheelmaker.py
@@ -260,7 +260,10 @@
     # add_wheelfile and add_metadata currently assume pure-Python.
     assert arguments.platform == 'any', "Only pure-Python wheels are supported"
 
-    input_files = [i.split(';') for i in arguments.input_file]
+    if arguments.input_file:
+        input_files = [i.split(';') for i in arguments.input_file]
+    else:
+        input_files = []
     all_files = get_files_to_package(input_files)
     # Sort the files for reproducible order in the archive.
     all_files = sorted(all_files.items())