elpaca--main-file: use regexp + directory-file search

Previous method of manually constructing a list of files was intended to be faster.
The performance gain in negligible and using a regexp + directory-files based
search is able to better find files in non-standard locations.

Author: progfolio

elpaca.el

@@ -1160,18 +1160,12 @@ The keyword's value is expected to be one of the following:
                      (declared (plist-member recipe :main)))
                 (cadr declared)
               (let* ((package (file-name-sans-extension (elpaca<-package e)))
-                     (filename (concat package ".el"))
-                     (regexp (concat "\\`" filename "//'")))
-                (or (cl-loop for name in (list (concat package "-pkg.el")
-                                               (concat "./lisp/" package "-pkg.el")
-                                               filename
-                                               (concat "./lisp/" filename)
-                                               (concat "./elisp/" filename))
-                             when (file-exists-p name) return name)
-                    (car (directory-files repo nil regexp))
-                    (car (elpaca--directory-files-recursively repo regexp))
-                    ;; Best guess if there is no file matching the package name...
-                    (car (directory-files repo nil "\\.el\\'" 'nosort))
+                     (name  (concat "\\(?:" package "\\(?:-pkg\\)?\\.el\\)\\'"))
+                     (prefix "\\`[^.z-a]*")
+                     (regexp (concat prefix name)))
+                (or (car (directory-files repo nil regexp))
+                    (car (cl-remove-if-not (apply-partially #'string-match-p (concat "[/\\]" name))
+                                           (elpaca--directory-files-recursively repo regexp)))
                     (error "Unable to find main elisp file for %S" package)))))))
 
 (defun elpaca--dependencies (e &optional recache)