Deprecate installing casks/formulae from paths.

We've already disabled installing casks/formulae from URLs and we
regularly tell people not to install from paths so let's just deprecate
this behaviour entirely.

Even Homebrew developers do not need to work this way.

Library/Homebrew/cask/cask_loader.rb

@@ -195,6 +195,8 @@ def initialize(url)
       def load(config:)
         path.dirname.mkpath
 
+        odeprecated "installing casks from paths or URLs", "installing casks from taps"
+
         if ALLOWED_URL_SCHEMES.exclude?(url.scheme)
           raise UnsupportedInstallationMethod,
                 "Non-checksummed download of #{name} formula file from an arbitrary URL is unsupported! " \

Library/Homebrew/formulary.rb

@@ -728,6 +728,8 @@ def initialize(url, from: nil)
       uri_path = URI(url).path
       raise ArgumentError, "URL has no path component" unless uri_path
 
+      odeprecated "installing formulae from paths or URLs", "installing formulae from taps"
+
       formula = File.basename(uri_path, ".rb")
       super formula, HOMEBREW_CACHE_FORMULA/File.basename(uri_path)
     end