add Tap#tap_migrations

This enables tap migration feature in per tap case, which will
ultimately help core/formula separation.

Library/Homebrew/core_formula_repository.rb

@@ -68,6 +68,12 @@ class CoreFormulaRepository < Tap
     FORMULA_RENAMES
   end
 
+  # @private
+  def tap_migrations
+    require "tap_migrations"
+    TAP_MIGRATIONS
+  end
+
   # @private
   def formula_file_to_name(file)
     file.basename(".rb").to_s

Library/Homebrew/tap.rb

@@ -76,6 +76,7 @@ class Tap
     @alias_reverse_table = nil
     @command_files = nil
     @formula_renames = nil
+    @tap_migrations = nil
   end
 
   # The remote path to this {Tap}.
@@ -370,6 +371,17 @@ class Tap
     end
   end
 
+  # Hash with tap migrations
+  def tap_migrations
+    require "utils/json"
+
+    @tap_migrations ||= if (migration_file = path/"tap_migrations.json").file?
+      Utils::JSON.load(migration_file.read)
+    else
+      {}
+    end
+  end
+
   def ==(other)
     other = Tap.fetch(other) if other.is_a?(String)
     self.class == other.class && self.name == other.name

Library/Homebrew/test/test_tap.rb

@@ -20,6 +20,9 @@ class TapTest < Homebrew::TestCase
     (@path/"formula_renames.json").write <<-EOS.undent
      { "oldname": "foo" }
     EOS
+    (@path/"tap_migrations.json").write <<-EOS.undent
+     { "removed-formula": "homebrew/foo" }
+    EOS
     @cmd_file = @path/"cmd/brew-tap-cmd.rb"
     @cmd_file.parent.mkpath
     FileUtils.touch @cmd_file
@@ -93,6 +96,7 @@ class TapTest < Homebrew::TestCase
     assert_equal @tap.alias_table, "homebrew/foo/bar" => "homebrew/foo/foo"
     assert_equal @tap.alias_reverse_table, "homebrew/foo/foo" => ["homebrew/foo/bar"]
     assert_equal @tap.formula_renames, "oldname" => "foo"
+    assert_equal @tap.tap_migrations, "removed-formula" => "homebrew/foo"
     assert_equal [@cmd_file], @tap.command_files
     assert_kind_of Hash, @tap.to_hash
     assert_equal true, @tap.formula_file?(@formula_file)