Use secondary check to see if Doom Emacs upgrade should be run

[mayersj1]

Topgrade should use a secondary check to determine if Doom Emacs upgrade should be run. Given that a default installation of Doom Emacs does NOT produce an init.el file in the ~/.emacs.d directory, the check for upgrading Emacs fails (which means it never attempts to detect if Doom Emacs is present or not. For Doom Emacs, this actually lives under ~/.doom.d/init.el.