Improve move_to_correct_build_directory

Why: Changes made are IMO much clearer, with a step by step flow to this
method that is easier to reason about.

src/pip/_internal/req/req_install.py

@@ -121,7 +121,6 @@ def __init__(
             markers = req.marker
         self.markers = markers
 
-        self._egg_info_path = None  # type: Optional[str]
         # This holds the pkg_resources.Distribution object if this requirement
         # is already available:
         self.satisfied_by = None
@@ -367,29 +366,34 @@ def move_to_correct_build_directory(self):
             return
         assert self.req is not None
         assert self._temp_build_dir
-        assert (self._ideal_build_dir is not None and
-                self._ideal_build_dir.path)  # type: ignore
+        assert (
+            self._ideal_build_dir is not None and
+            self._ideal_build_dir.path  # type: ignore
+        )
         old_location = self._temp_build_dir
-        self._temp_build_dir = None
+        self._temp_build_dir = None  # checked inside ensure_build_location
 
+        # Figure out the correct place to put the files.
         new_location = self.ensure_build_location(self._ideal_build_dir)
         if os.path.exists(new_location):
             raise InstallationError(
                 'A package already exists in %s; please remove it to continue'
-                % display_path(new_location))
+                % display_path(new_location)
+            )
+
+        # Move the files to the correct location.
         logger.debug(
             'Moving package %s from %s to new location %s',
             self, display_path(old_location.path), display_path(new_location),
         )
         shutil.move(old_location.path, new_location)
+
+        # Update directory-tracking variables, to be in line with new_location
+        self.source_dir = os.path.normpath(os.path.abspath(new_location))
         self._temp_build_dir = TempDirectory(
             path=new_location, kind="req-install",
         )
 
-        self._ideal_build_dir = None
-        self.source_dir = os.path.normpath(os.path.abspath(new_location))
-        self._egg_info_path = None
-
         # Correct the metadata directory, if it exists
         if self.metadata_directory:
             old_meta = self.metadata_directory
@@ -398,6 +402,11 @@ def move_to_correct_build_directory(self):
             new_meta = os.path.normpath(os.path.abspath(new_meta))
             self.metadata_directory = new_meta
 
+        # Done with any "move built files" work, since have moved files to the
+        # "ideal" build location. Setting to None allows to clearly flag that
+        # no more moves are needed.
+        self._ideal_build_dir = None
+
     def remove_temporary_source(self):
         # type: () -> None
         """Remove the source files from this requirement, if they are marked