enumerate all formulae by files rather than names

Before
```
$ time brew readall
brew readall  10.63s user 0.36s system 99% cpu 11.003 total
```

After
```
$ time brew readall
brew readall  5.62s user 0.24s system 99% cpu 5.859 total
```

Closes Homebrew/homebrew#42302.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/readall.rb

@@ -32,18 +32,18 @@ module Homebrew
 
     formulae = []
     if ARGV.named.empty?
-      formulae = Formula.full_names
+      formulae = Formula.files
     else
       tap = Tap.new(*tap_args)
       raise TapUnavailableError, tap.name unless tap.installed?
       formulae = tap.formula_files
     end
 
-    formulae.sort.each do |n|
+    formulae.each do |file|
       begin
-        Formulary.factory(n)
+        Formulary.factory(file)
       rescue Exception => e
-        onoe "problem in #{Formulary.path(n)}"
+        onoe "problem in #{file}"
         puts e
         Homebrew.failed = true
       end

Library/Homebrew/formula.rb

@@ -655,28 +655,43 @@ class Formula
     @core_names ||= Dir["#{HOMEBREW_LIBRARY}/Formula/*.rb"].map{ |f| File.basename f, ".rb" }.sort
   end
 
+  # an array of all core {Formula} files
+  def self.core_files
+    @core_files ||= Pathname.glob("#{HOMEBREW_LIBRARY}/Formula/*.rb")
+  end
+
   # an array of all tap {Formula} names
   def self.tap_names
     @tap_names ||= Tap.map(&:formula_names).flatten.sort
   end
 
+  # an array of all tap {Formula} files
+  def self.tap_files
+    @tap_files ||= Tap.map(&:formula_files).flatten
+  end
+
   # an array of all {Formula} names
   def self.names
     @names ||= (core_names + tap_names.map { |name| name.split("/")[-1] }).sort.uniq
   end
 
+  # an array of all {Formula} files
+  def self.files
+    @files ||= core_files + tap_files
+  end
+
   # an array of all {Formula} names, which the tap formulae have the fully-qualified name
   def self.full_names
     @full_names ||= core_names + tap_names
   end
 
   def self.each
-    full_names.each do |name|
+    files.each do |file|
       begin
-        yield Formulary.factory(name)
+        yield Formulary.factory(file)
       rescue StandardError => e
         # Don't let one broken formula break commands. But do complain.
-        onoe "Failed to import: #{name}"
+        onoe "Failed to import: #{file}"
         puts e
         next
       end