[rubygems/rubygems] Reduce allocations when installing gems with bundler

```
==> memprof.after.txt <==
Total allocated: 1.13 MB (2352 objects)
Total retained:  10.08 kB (78 objects)

==> memprof.before.txt <==
Total allocated: 46.27 MB (38439 objects)
Total retained:  9.94 kB (75 objects)
```

Yes, we were allocating 45MB of arrays in `dependencies_installed?`,
it was accidentally cubic.

rubygems/rubygems@13ab874388

lib/bundler/installer/parallel_installer.rb

@@ -42,8 +42,7 @@ def ignorable_dependency?(dep)
 
       # Checks installed dependencies against spec's dependencies to make
       # sure needed dependencies have been installed.
-      def dependencies_installed?(all_specs)
-        installed_specs = all_specs.select(&:installed?).map(&:name)
+      def dependencies_installed?(installed_specs)
         dependencies.all? {|d| installed_specs.include? d.name }
       end
 
@@ -183,8 +182,14 @@ def require_tree_for_spec(spec)
     # previously installed specifications. We continue until all specs
     # are installed.
     def enqueue_specs
-      @specs.select(&:ready_to_enqueue?).each do |spec|
-        if spec.dependencies_installed? @specs
+      installed_specs = {}
+      @specs.each do |spec|
+        next unless spec.installed?
+        installed_specs[spec.name] = true
+      end
+
+      @specs.each do |spec|
+        if spec.ready_to_enqueue? && spec.dependencies_installed?(installed_specs)
           spec.state = :enqueued
           worker_pool.enq spec
         end

spec/bundler/bundler/installer/spec_installation_spec.rb

@@ -47,7 +47,8 @@ def a_spec.full_name
         all_specs = dependencies + [instance_double("SpecInstallation", :spec => "gamma", :name => "gamma", :installed? => false, :all_dependencies => [], :type => :production)]
         spec = described_class.new(dep)
         allow(spec).to receive(:all_dependencies).and_return(dependencies)
-        expect(spec.dependencies_installed?(all_specs)).to be_truthy
+        installed_specs = all_specs.select(&:installed?).map {|s| [s.name, true] }.to_h
+        expect(spec.dependencies_installed?(installed_specs)).to be_truthy
       end
     end
 
@@ -59,7 +60,8 @@ def a_spec.full_name
         all_specs = dependencies + [instance_double("SpecInstallation", :spec => "gamma", :name => "gamma", :installed? => false, :all_dependencies => [], :type => :production)]
         spec = described_class.new(dep)
         allow(spec).to receive(:all_dependencies).and_return(dependencies)
-        expect(spec.dependencies_installed?(all_specs)).to be_falsey
+        installed_specs = all_specs.select(&:installed?).map {|s| [s.name, true] }.to_h
+        expect(spec.dependencies_installed?(installed_specs)).to be_falsey
       end
     end
   end