Revamp the setup script's install command

Previously, the Hy files would be bytecode compiled before the
compiler's dependencies were installed. In additon, the revamped
version properly propogates the optimization level and generally is a
bit cleaner.

Fixes #1864.

LICENSE

@@ -1,4 +1,5 @@
 Copyright 2020 the authors.
+Portions of setup.py, copyright 2016 Jason R Coombs <jaraco@jaraco.com>.
 
 Permission is hereby granted, free of charge, to any person obtaining a
 copy of this software and associated documentation files (the "Software"),

setup.py

@@ -3,7 +3,11 @@
 # This file is part of Hy, which is free software licensed under the Expat
 # license. See the LICENSE.
 
-import sys, os
+import glob
+import importlib
+import inspect
+import os
+import sys
 
 from setuptools import find_packages, setup
 from setuptools.command.install import install
@@ -20,16 +24,42 @@ make things work nicer, and lets Python and the Hy lisp variant play
 nice together. """
 
 class Install(install):
+    def __compile_hy_bytecode(self):
+        for path in sorted(glob.iglob('hy/**.hy', recursive=True)):
+            importlib.util.cache_from_source(path, optimize=self.optimize)
+
     def run(self):
-        # Import each Hy module to ensure it's compiled.
-        import os, importlib
-        for dirpath, _, filenames in sorted(os.walk("hy")):
-            for filename in sorted(filenames):
-                if filename.endswith(".hy"):
-                    importlib.import_module(
-                        dirpath.replace("/", ".").replace("\\", ".") +
-                        "." + filename[:-len(".hy")])
-        install.run(self)
+        # Don't bother messing around with deps if they wouldn't be installed anyway.
+        # Code is based on setuptools's install.py.
+        if not (self.old_and_unmanageable or self.single_version_externally_managed
+                or not self._called_from_setup(inspect.currentframe())):
+            easy_install = self.distribution.get_command_class('easy_install')
+
+            cmd = easy_install(
+                self.distribution, args="x", root=self.root, record=self.record,
+            )
+            cmd.ensure_finalized()
+            cmd.always_copy_from = '.'
+            cmd.package_index.scan(glob.glob('*.egg'))
+
+            cmd.args = self.distribution.install_requires
+
+            # Avoid deprecation warnings on new setuptools versions.
+            if 'show_deprecation' in inspect.signature(cmd.run).parameters:
+                cmd.run(show_deprecation=False)
+            else:
+                cmd.run()
+
+            # Make sure any new packages get picked up.
+            import site
+            importlib.reload(site)
+            importlib.invalidate_caches()
+
+        self.__compile_hy_bytecode()
+
+        # The deps won't be reinstalled because of:
+        # https://github.com/pypa/setuptools/issues/456
+        return install.run(self)
 
 install_requires = [
     'rply>=0.7.7',