Fix pypa#104 (path pollution)

_in_process.py is executed as a subprocess. This causes its parent
directory to become sys.path[0]. Any extra files in the same directory
shadow pypi libraries eg colorlog.

Solution: put _in_process.py in its own special subdirectory.
Note: the directory has to be a module because
importlib.resources.path does not support resources with directory
separators in their name:

https://docs.python.org/3/library/importlib.html#importlib.resources.path :
> resource is the name of the resource to open within package;
> it may not contain path separators and it may not have sub-resources
> (i.e. it cannot be a directory).

pep517/in_process/__init__.py

@@ -0,0 +1,12 @@
+from os.path import dirname, abspath, join as pjoin
+from contextlib import contextmanager
+
+try:
+    import importlib.resources as resources
+
+    def _in_proc_script_path():
+        return resources.path(__package__, '_in_process.py')
+except ImportError:
+    @contextmanager
+    def _in_proc_script_path():
+        yield pjoin(dirname(abspath(__file__)), '_in_process.py')

pep517/wrappers.py

@@ -8,6 +8,7 @@
 from tempfile import mkdtemp
 
 from . import compat
+from .in_process import _in_proc_script_path
 
 __all__ = [
     'BackendUnavailable',
@@ -19,16 +20,6 @@
     'Pep517HookCaller',
 ]
 
-try:
-    import importlib.resources as resources
-
-    def _in_proc_script_path():
-        return resources.path(__package__, '_in_process.py')
-except ImportError:
-    @contextmanager
-    def _in_proc_script_path():
-        yield pjoin(dirname(abspath(__file__)), '_in_process.py')
-
 
 @contextmanager
 def tempdir():