Improve counsel-projectile-find-file performance

Current code will show significant lag for projects with large numbers
of files.  Tests with 400K files shows 15-20s lag for
counsel-projectile-find-file.  Profiling showed that this was
due to the preprocessing of projectile-current-project-files to add
'ivy-virtual face to files not already in a buffer.  To fix this,
a display transformer was added for this command so that the face
modification was applied only to at most ivy-height files.

The preprocessed file list was also used by counsel-projectile
code paths, so some additional modifications were made to ensure
that that code path does not change behavior.  Any similar issues in
counsel-projectile performance was not addressed in this commit.

Details:

* move 'ivy-virtual face change to a display transformer function
* use projectile-current-project-files instead of counsel-projectile--file-list
  for ivy-read.
* reduce the scope of counsel-projectile--file-list, making it return
  only unvisited files, and renaming it accordingly.  This is now only
  used indirectly by the counsel-projectile function.
* change ivy-switch-buffer-transformer to propertize files at display time.

counsel-projectile.el

@@ -65,20 +65,16 @@
 
 ;;; counsel-projectile-find-file
 
-(defun counsel-projectile--file-list (&optional no-buffer)
-  "Return a list of files for the current project.
+(defun counsel-projectile--unvisited-file-list ()
+  "Return a list of unvisited files for the current project.
 
-Like `projectile-current-project-files', but fontifies
-non-visited file names with the `ivy-virtual' face.  With optional
-argument NO-BUFFER, only list non-visited files."
+Like `projectile-current-project-files', but skips any files
+already being visited by a buffer."
   (let ((root (projectile-project-root)))
     (cl-loop
      for name in (projectile-current-project-files)
      for file = (expand-file-name name root)
      if (not (get-file-buffer file))
-     collect (propertize name 'face 'ivy-virtual)
-     else
-     unless no-buffer
      collect name)))
 
 (defun counsel-projectile--find-file-action (file &optional other-window)
@@ -94,6 +90,12 @@ argument NO-BUFFER, only list non-visited files."
 `projectile-find-file-hook'."
   (counsel-projectile--find-file-action file t))
 
+(defun counsel-projectile--find-file-transformer (name)
+  "Transform non-visited file names with `ivy-virtual' face."
+  (if (not (get-file-buffer (expand-file-name name (projectile-project-root))))
+      (propertize name 'face 'ivy-virtual)
+    name))
+
 ;;;###autoload
 (defun counsel-projectile-find-file (&optional arg)
   "Jump to a project's file using completion.
@@ -103,7 +105,7 @@ invalidates the cache first."
   (interactive "P")
   (projectile-maybe-invalidate-cache arg)
   (ivy-read (projectile-prepend-project-name "Find file: ")
-            (counsel-projectile--file-list)
+            (projectile-current-project-files)
             :matcher #'counsel--find-file-matcher
             :require-match t
             :keymap counsel-projectile-map
@@ -115,6 +117,10 @@ invalidates the cache first."
  '(("j" counsel-projectile--find-file-other-window-action
     "other window")))
 
+(ivy-set-display-transformer
+ 'counsel-projectile-find-file
+ 'counsel-projectile--find-file-transformer)
+
 ;;; counsel-projectile-find-dir
 
 (defun counsel-projectile--dir-list ()
@@ -370,15 +376,17 @@ invokes `projectile-commander' instead of
          (counsel-projectile--buffer-list))
    (mapc (lambda (file)
            (add-text-properties 0 1 '(type file) file))
-         (counsel-projectile--file-list t))))
+         (counsel-projectile--unvisited-file-list))))
 
 (defun counsel-projectile--transformer (str)
   "Fontifies modified, file-visiting buffers.
 
 Relies on `ivy-switch-buffer-transformer'."
-  (if (eq (get-text-property 0 'type str) 'buffer)
-      (ivy-switch-buffer-transformer str)
-    str))
+  (let ((type (get-text-property 0 'type str)))
+    (cond
+     ((eq type 'buffer) (ivy-switch-buffer-transformer str))
+     ((eq type 'file) (propertize str 'face 'ivy-virtual))
+     (t str))))
 
 (defun counsel-projectile--matcher (regexp candidates)
   "Return REGEXP-matching CANDIDATES.