cleanup: allow users to specify formulae to skip cleaning

This allows users to set `HOMEBREW_NO_CLEANUP_FORMULAE` to a
comma-separated list of formulae that `brew` will refuse to clean with
`brew cleanup`.

We currently allow a less granular version of this with
`HOMEBREW_NO_INSTALL_CLEANUP`. All this changes is how much control
users have over what is and isn't cleaned.

Fixes #11924.

Library/Homebrew/cleanup.rb

@@ -160,11 +160,18 @@ module Homebrew
       cleanup = Cleanup.new
       if cleanup.periodic_clean_due?
         cleanup.periodic_clean!
-      elsif f.latest_version_installed?
+      elsif f.latest_version_installed? && !cleanup.skip_clean_formula?(f)
         cleanup.cleanup_formula(f)
       end
     end
 
+    def skip_clean_formula?(f)
+      return false if Homebrew::EnvConfig.no_cleanup_formulae.blank?
+
+      skip_clean_formulae = Homebrew::EnvConfig.no_cleanup_formulae.split(",")
+      skip_clean_formulae.include?(f.name) || (skip_clean_formulae & f.aliases).present?
+    end
+
     def periodic_clean_due?
       return false if Homebrew::EnvConfig.no_install_cleanup?
 
@@ -186,7 +193,10 @@ module Homebrew
 
     def clean!(quiet: false, periodic: false)
       if args.empty?
-        Formula.installed.sort_by(&:name).each do |formula|
+        Formula.installed
+               .sort_by(&:name)
+               .reject { |f| skip_clean_formula?(f) }
+               .each do |formula|
           cleanup_formula(formula, quiet: quiet, ds_store: false, cache_db: false)
         end
         cleanup_cache
@@ -223,7 +233,12 @@ module Homebrew
             nil
           end
 
-          cleanup_formula(formula) if formula
+          if formula && skip_clean_formula?(formula)
+            onoe "Refusing to clean #{formula} because it is listed in " \
+                 "#{Tty.bold}HOMEBREW_NO_CLEANUP_FORMULAE#{Tty.reset}!"
+          elsif formula
+            cleanup_formula(formula)
+          end
           cleanup_cask(cask) if cask
         end
       end

Library/Homebrew/env_config.rb

@@ -229,6 +229,10 @@ module Homebrew
                      "formulae.",
         boolean:     true,
       },
+      HOMEBREW_NO_CLEANUP_FORMULAE:           {
+        description: "A comma-separated list of formulae. Homebrew will refuse to clean up a " \
+                     "formula if it appears on this list.",
+      },
       HOMEBREW_NO_COLOR:                      {
         description:  "If set, do not print text with colour added.",
         default_text: "`$NO_COLOR`.",