! Add test and fix installer dep finding bug

The previous commit removed changing the global paths when doing an
install, which caused the installer to not be able to find gems inside
the install directory. This test for some reason didn't fail though.

lib/rubygems/installer.rb

@@ -285,6 +285,24 @@ def install
     raise Gem::InstallError, "gzip error installing #{gem}"
   end
 
+  ##
+  #
+  # Return an Array of Specifications contained within the gem_home
+  # we'll be installing into.
+
+  def installed_specs
+    @specs ||= begin
+      specs = []
+
+      Dir[File.join(gem_home, "specifications", "*.gemspec")].each do |path|
+        spec = Gem::Specification.load path.untaint
+        specs << spec if spec
+      end
+
+      specs
+    end
+  end
+
   ##
   # Ensure that the dependency is satisfied by the current installation of
   # gem.  If it is not an exception is raised.
@@ -303,6 +321,7 @@ def ensure_dependency(spec, dependency)
   # True if the gems in the source_index satisfy +dependency+.
 
   def installation_satisfies_dependency?(dependency)
+    return true if installed_specs.detect { |s| dependency.matches_spec? s }
     not dependency.matching_specs.empty?
   end
 

test/rubygems/test_gem_installer.rb

@@ -791,13 +791,15 @@ def test_install_check_dependencies
 
   def test_install_check_dependencies_install_dir
     gemhome2 = "#{@gemhome}2"
-    @spec.add_dependency 'b'
+    @spec.add_dependency 'd'
 
-    quick_gem 'b', 2
+    quick_gem 'd', 2
 
     FileUtils.mv @gemhome, gemhome2
 
-    Gem::Specification.dirs = [gemhome2] # TODO: switch all dirs= to use_paths
+    # Don't leak any already activated gems into the installer, require
+    # that it work everything out on it's own.
+    Gem::Specification.reset
 
     util_setup_gem