Generate warning when local repository names are in conflict

[raxod502]

Currently, if you use two packages from MELPA (or any source) which result in the same auto-generated local repository name (:local-repo), e.g.

• https://github.com/eschulte/emacs-web-server (web-server)
• https://github.com/skeeto/emacs-web-server (simple-httpd)

then straight.el generates a warning

Warning (straight): Packages "web-server" and "simple-httpd" have incompatible recipes (:repo cannot be both "eschulte/emacs-web-server" and "skeeto/emacs-web-server")

However, this is not a very informative warning to the user, because it does not suggest the resolution of the issue, which is to manually specify :local-repo for one of the recipes. A better warning should be shown.

Related issues:

• Can't handle different package with the same repo name ? #265
• repos with the same name conflict, but shouldn't #495