Restore deleted _copy_dist_from_dir().

This reverts commit 62ac258.

#3176 is about to add the missing piece that makes this code useful (and not dead), so let's not delete it.

pip/download.py

@@ -702,6 +702,45 @@ def unpack_file_url(link, location, download_dir=None, hashes=None):
         _copy_file(from_path, download_dir, content_type, link)
 
 
+def _copy_dist_from_dir(link_path, location):
+    """Copy distribution files in `link_path` to `location`.
+
+    Invoked when user requests to install a local directory. E.g.:
+
+        pip install .
+        pip install ~/dev/git-repos/python-prompt-toolkit
+
+    """
+
+    # Note: This is currently VERY SLOW if you have a lot of data in the
+    # directory, because it copies everything with `shutil.copytree`.
+    # What it should really do is build an sdist and install that.
+    # See https://github.com/pypa/pip/issues/2195
+
+    if os.path.isdir(location):
+        rmtree(location)
+
+    # build an sdist
+    setup_py = 'setup.py'
+    sdist_args = [sys.executable]
+    sdist_args.append('-c')
+    sdist_args.append(
+        "import setuptools, tokenize;__file__=%r;"
+        "exec(compile(getattr(tokenize, 'open', open)(__file__).read()"
+        ".replace('\\r\\n', '\\n'), __file__, 'exec'))" % setup_py)
+    sdist_args.append('sdist')
+    sdist_args += ['--dist-dir', location]
+    logger.info('Running setup.py sdist for %s', link_path)
+
+    with indent_log():
+        call_subprocess(sdist_args, cwd=link_path, show_stdout=False)
+
+    # unpack sdist into `location`
+    sdist = os.path.join(location, os.listdir(location)[0])
+    logger.info('Unpacking sdist %s into %s', sdist, location)
+    unpack_file(sdist, location, content_type=None, link=None)
+
+
 class PipXmlrpcTransport(xmlrpc_client.Transport):
     """Provide a `xmlrpclib.Transport` implementation via a `PipSession`
     object.