Distutils script: supporting future and docstrings.

Fixes #102

Distutils scripts can contain quite a lot of code. Up till now,
only a encoding hint was detected and placed at the top above
our 'import sys' and 'sys.path=' stuff.

Now we look for the first non-__future__ import line and place our
sys.path statement above that line. This makes sure the encoding hint,
the module docstring and the future imports are all above our code,
as should be.

There might be corner cases, but at least this fixes a couple of
common problems.

src/zc/buildout/easy_install.py

@@ -1067,27 +1067,36 @@ def _script(module_name, attrs, path, dest, arguments, initialization, rsetup):
 def _distutils_script(path, dest, script_content, initialization, rsetup):
     if is_win32:
         dest += '-script.py'
-
     lines = script_content.splitlines(True)
     if not ('#!' in lines[0]) and ('python' in lines[0]):
         # The script doesn't follow distutil's rules.  Ignore it.
         return []
-    source_encoding_line = ''
-    original_content = ''.join(lines[1:])
-    if (len(lines) > 1) and is_source_encoding_line(lines[1]):
-        # The second line contains a source encoding line. Copy it verbatim.
-        source_encoding_line = lines[1].rstrip()
-        original_content = ''.join(lines[2:])
+    lines = lines[1:]  # Strip off the first hashbang line.
+    line_with_first_import = None
+    for line_number, line in enumerate(lines):
+        if not 'import' in line:
+            continue
+        if not line.startswith('import') or line.startswith('from'):
+            continue
+        if '__future__' in line:
+            continue
+        line_with_first_import = line_number
+    if line_with_first_import is None:
+        before = ''.join(lines)
+        after = ''
+    else:
+        before = ''.join(lines[:line_with_first_import])
+        after = ''.join(lines[line_with_first_import:])
 
     python = _safe_arg(sys.executable)
 
     contents = distutils_script_template % dict(
         python = python,
-        source_encoding_line = source_encoding_line,
         path = path,
         initialization = initialization,
         relative_paths_setup = rsetup,
-        original_content = original_content
+        before = before,
+        after = after
         )
     return _create_script(contents, dest)
 
@@ -1148,9 +1157,8 @@ if __name__ == '__main__':
     sys.exit(%(module_name)s.%(attrs)s(%(arguments)s))
 '''
 
-distutils_script_template = script_header + '''\
-
-%(source_encoding_line)s
+distutils_script_template = script_header + '''
+%(before)s
 %(relative_paths_setup)s
 import sys
 sys.path[0:0] = [
@@ -1158,7 +1166,7 @@ sys.path[0:0] = [
   ]
 %(initialization)s
 
-%(original_content)s'''
+%(after)s'''
 
 
 def _pyscript(path, dest, rsetup, initialization=''):