pypy: install to libexec and install pip

* sequester pypy in `libexec` and symlink out its binaries only
* re-add pip installation to pypy

Fixes Homebrew/homebrew#20809.
Closes Homebrew/homebrew#22807.

Formula/pypy.rb

@@ -13,10 +13,19 @@ class Pypy < Formula
     sha1 'b8bf9c2b8a114045598f0e16681d6a63a4d6cdf9'
   end
 
+  resource 'pip' do
+    url 'https://pypi.python.org/packages/source/p/pip/pip-1.4.1.tar.gz'
+    sha1 '9766254c7909af6d04739b4a7732cc29e9a48cb0'
+  end
+
   def install
     rmtree 'site-packages'
 
-    prefix.install Dir['*']
+    # The PyPy binary install instructions suggest installing somewhere
+    # (like /opt) and symlinking in binaries as needed. Specifically,
+    # we want to avoid putting PyPy's Python.h somewhere that configure
+    # scripts will find it.
+    libexec.install Dir['*']
 
     # Post-install, fix up the site-packages and install-scripts folders
     # so that user-installed Python software survives minor updates, such
@@ -26,7 +35,7 @@ class Pypy < Formula
     prefix_site_packages.mkpath
 
     # Symlink the prefix site-packages into the cellar.
-    ln_s prefix_site_packages, prefix+'site-packages'
+    ln_s prefix_site_packages, libexec+'site-packages'
 
     # Tell distutils-based installers where to put scripts
     scripts_folder.mkpath
@@ -39,7 +48,8 @@ class Pypy < Formula
     # $ easy_install pip
     # $ pip install --upgrade setuptools
     # to get newer versions of setuptools outside of Homebrew.
-    resource('setuptools').stage { system "#{bin}/pypy", "setup.py", "install" }
+    resource('setuptools').stage { system "#{libexec}/bin/pypy", "setup.py", "install" }
+    resource('pip').stage { system "#{libexec}/bin/pypy", "setup.py", "install" }
 
     # Symlink to easy_install_pypy.
     unless (scripts_folder+'easy_install_pypy').exist?
@@ -84,6 +94,6 @@ class Pypy < Formula
 
   # The Cellar location of distutils
   def distutils
-    prefix+"lib-python/2.7/distutils"
+    libexec+"lib-python/2.7/distutils"
   end
 end