Confirm that pyc files are written during installation

In order to add generated pyc files to the RECORD file for our package,
we need to know their path! To raise confidence that we're doing this
correctly, we assert the existence of the expected 'pyc' files while
still using the old installation logic.

Some valid reasons why pyc files may not be generated:

1. Syntax error in the installed Python files
2. There is already a pyc file in-place that isn't writable by the
   current user

We don't fail installation in those cases today, and we wouldn't want to
change our behavior here, so we only assert that the pyc file was
created if `compileall.compile_file` indicates success.

src/pip/_internal/operations/install/wheel.py

@@ -7,6 +7,7 @@ import collections
 import compileall
 import contextlib
 import csv
+import importlib
 import logging
 import os.path
 import re
@@ -467,6 +468,18 @@ def install_unpacked_wheel(
                     continue
                 yield os.path.join(dir_path, path)
 
+    def pyc_output_path(path):
+        # type: (text_type) -> text_type
+        """Return the path the pyc file would have been written to.
+        """
+        if PY2:
+            if sys.flags.optimize:
+                return path + 'o'
+            else:
+                return path + 'c'
+        else:
+            return importlib.util.cache_from_source(path)
+
     # Compile all of the pyc files that we're going to be installing
     if pycompile:
         with captured_stdout() as stdout:
@@ -478,9 +491,11 @@ def install_unpacked_wheel(
                     path_arg = ensure_str(
                         path, encoding=sys.getfilesystemencoding()
                     )
-                    compileall.compile_file(
+                    success = compileall.compile_file(
                         path_arg, force=True, quiet=True
                     )
+                    if success:
+                        assert os.path.exists(pyc_output_path(path))
         logger.debug(stdout.getvalue())
 
     def record_installed(srcfile, destfile, modified=False):