requirements: split internal and external requirements in two separate files

MANIFEST.in

 include Makefile
 include README
 include requirements.txt
+include requirements-swh.txt
 include version.txt

requirements-swh.txt

+swh.core
+swh.storage >= 0.0.76
+swh.scheduler >= 0.0.10

requirements.txt

 requests
 SQLAlchemy
 
-swh.core
-swh.storage >= 0.0.76
-swh.scheduler >= 0.0.10

setup.py

@@ -5,13 +5,13 @@ from setuptools import setup
 
 def parse_requirements():
     requirements = []
-    with open('requirements.txt') as f:
-        for line in f.readlines():
-            line = line.strip()
-            if not line or line.startswith('#'):
-                continue
-            requirements.append(line)
-
+    for reqf in ('requirements.txt', 'requirements-swh.txt'):
+        with open(reqf) as f:
+            for line in f.readlines():
+                line = line.strip()
+                if not line or line.startswith('#'):
+                    continue
+                requirements.append(line)
     return requirements