remove extra caching on spec

The package object already caches the spec object, so we don't need to
do it multiple times.  Also introduce a consturctor for in-memory
installations so that we can rm the `spec` setter method

lib/rubygems/installer.rb

@@ -61,11 +61,6 @@ class Gem::Installer
 
   attr_reader :options
 
-  ##
-  # Sets the specification for .gem-less installs.
-
-  attr_writer :spec
-
   @path_warning = false
 
   @install_lock = Mutex.new
@@ -108,6 +103,15 @@ def self.at path, options = {}
     new package, options
   end
 
+  ##
+  # Construct an installer object for an ephemeral gem (one where we don't
+  # actually have a .gem file, just a spec)
+
+  def self.for_spec spec, options = {}
+    # FIXME: we should have a real Package class for this
+    new Struct.new(:spec).new(spec), options
+  end
+
   ##
   # Constructs an Installer instance that will install the gem located at
   # +gem+.  +options+ is a Hash with the following keys:
@@ -225,7 +229,7 @@ def gem_dir
   # Lazy accessor for the installer's spec.
 
   def spec
-    @spec ||= @package.spec
+    @package.spec
   rescue Gem::Package::Error => e
     raise Gem::InstallError, "invalid gem: #{e.message}"
   end
@@ -244,7 +248,7 @@ def spec
   def install
     pre_install_checks
 
-    FileUtils.rm_f File.join gem_home, 'specifications', @spec.spec_name
+    FileUtils.rm_f File.join gem_home, 'specifications', spec.spec_name
 
     run_pre_install_hooks
 

lib/rubygems/resolver/git_specification.rb

@@ -23,8 +23,7 @@ def add_dependency dependency # :nodoc:
   def install options = {}
     require 'rubygems/installer'
 
-    installer = Gem::Installer.at '', options
-    installer.spec = spec
+    installer = Gem::Installer.for_spec spec, options
 
     yield installer if block_given?
 

test/rubygems/test_gem_installer.rb

@@ -575,8 +575,8 @@ def test_generate_bin_symlink_update_older
     util_make_exec
     one = @spec.dup
     one.version = 1
+    @installer = Gem::Installer.for_spec spec
     @installer.gem_dir = util_gem_dir one
-    @installer.spec = spec
 
     @installer.generate_bin
 
@@ -1427,7 +1427,6 @@ def test_write_cache_file
     refute_path_exists cache_file
 
     installer = Gem::Installer.at gem
-    installer.spec = @spec
     installer.gem_home = @gemhome
 
     installer.write_cache_file
@@ -1439,7 +1438,7 @@ def test_write_spec
     FileUtils.rm @spec.spec_file
     refute_path_exists @spec.spec_file
 
-    @installer.spec = @spec
+    @installer = Gem::Installer.for_spec @spec
     @installer.gem_home = @gemhome
 
     @installer.write_spec
@@ -1459,7 +1458,7 @@ def test_write_spec_writes_cached_spec
 
     @spec.files = %w[a.rb b.rb c.rb]
 
-    @installer.spec = @spec
+    @installer = Gem::Installer.for_spec @spec
     @installer.gem_home = @gemhome
 
     @installer.write_spec