Allow the debugger to work from inside staged resources

References Homebrew/homebrew#23263.
Closes Homebrew/homebrew#27445.

Library/Homebrew/build.rb

@@ -167,6 +167,8 @@ class Build
       else
         f.prefix.mkpath
 
+        f.resources.each { |r| r.extend(ResourceDebugger) } if ARGV.debug?
+
         begin
           f.install
 

Library/Homebrew/debrew.rb

@@ -1,5 +1,6 @@
 require 'debrew/menu'
 require 'debrew/raise_plus'
+require 'set'
 
 unless ENV['HOMEBREW_NO_READLINE']
   begin
@@ -15,7 +16,27 @@ class Object
   include RaisePlus
 end
 
+module ResourceDebugger
+  def stage(&block)
+    super do
+      begin
+        block.call(self)
+      rescue Exception => e
+        if ARGV.debug?
+          debrew e
+        else
+          raise
+        end
+      end
+    end
+  end
+end
+
+$debugged_exceptions = Set.new
+
 def debrew(exception, formula=nil)
+  raise exception unless $debugged_exceptions.add?(exception)
+
   puts "#{exception.backtrace.first}"
   puts "#{Tty.red}#{exception.class.to_s}#{Tty.reset}: #{exception.to_s}"