Add experimental support for lazy loading the info extractors

'make lazy-extractors' creates the youtube_dl/extractor/lazy_extractors.py (imported by youtube_dl/extractor/__init__.py), which contains simplified classes that only have the 'suitable' class method and that load the appropiate class with the '__new__' method when a instance is created.

devscripts/lazy_load_template.py

@@ -0,0 +1,17 @@
+# flake8: noqa
+from __future__ import unicode_literals
+
+import re
+
+
+class LazyLoadExtractor(object):
+    _module = None
+
+    @classmethod
+    def ie_key(cls):
+        return cls.__name__[:-2]
+
+    def __new__(cls):
+        mod = __import__(cls._module, fromlist=(cls.__name__,))
+        real_cls = getattr(mod, cls.__name__)
+        return real_cls.__new__(real_cls)

devscripts/make_lazy_extractors.py

@@ -0,0 +1,63 @@
+from __future__ import unicode_literals, print_function
+
+from inspect import getsource
+import os
+from os.path import dirname as dirn
+import sys
+
+print('WARNING: Lazy loading extractors is an experimental feature that may not always work', file=sys.stderr)
+
+sys.path.insert(0, dirn(dirn((os.path.abspath(__file__)))))
+
+lazy_extractors_filename = sys.argv[1]
+if os.path.exists(lazy_extractors_filename):
+    os.remove(lazy_extractors_filename)
+
+from youtube_dl.extractor import _ALL_CLASSES
+from youtube_dl.extractor.common import InfoExtractor
+
+with open('devscripts/lazy_load_template.py', 'rt') as f:
+    module_template = f.read()
+
+module_contents = [module_template + '\n' + getsource(InfoExtractor.suitable)]
+
+ie_template = '''
+class {name}(LazyLoadExtractor):
+    _VALID_URL = {valid_url!r}
+    _module = '{module}'
+'''
+
+make_valid_template = '''
+    @classmethod
+    def _make_valid_url(cls):
+        return {!r}
+'''
+
+
+def build_lazy_ie(ie, name):
+    valid_url = getattr(ie, '_VALID_URL', None)
+    s = ie_template.format(
+        name=name,
+        valid_url=valid_url,
+        module=ie.__module__)
+    if ie.suitable.__func__ is not InfoExtractor.suitable.__func__:
+        s += getsource(ie.suitable)
+    if hasattr(ie, '_make_valid_url'):
+        # search extractors
+        s += make_valid_template.format(ie._make_valid_url())
+    return s
+
+names = []
+for ie in _ALL_CLASSES:
+    name = ie.ie_key() + 'IE'
+    src = build_lazy_ie(ie, name)
+    module_contents.append(src)
+    names.append(name)
+
+module_contents.append(
+    '_ALL_CLASSES = [{}]'.format(', '.join(names)))
+
+module_src = '\n'.join(module_contents)
+
+with open(lazy_extractors_filename, 'wt') as f:
+    f.write(module_src)