Make pbr a build-time only dependency

This lets you build swift packages that don't require pbr
to be installed at all. You would need pbr on the machine running
rpmbuild / debuild, but not on the machines that install the packages.

Unfortunately, this does not make swift able to be
installed via pip 0.3.1 on Lucid; you'll need to uninstall the system
python-pip package and install a new pip some other way. Given that
pip < 1.3 doesn't perform SSL certificate validation for pypi (trivial
MITM attack, anyone?), you'd probably want to get a new pip anyway.

Change-Id: Ia50a229c5ae4dd2158beeaa953619b5e8f987c55

requirements.txt

@@ -1,4 +1,3 @@
-pbr>=0.5.21,<1.0
 dnspython>=1.9.4
 eventlet>=0.9.15
 greenlet>=0.3.1

swift/__init__.py

@@ -16,13 +16,22 @@
 import os
 import gettext
 
-import pbr.version
-
-
-_version_info = pbr.version.VersionInfo('swift')
-__version__ = _version_info.release_string()
-__canonical_version__ = _version_info.version_string()
+import pkg_resources
 
+try:
+    # First, try to get our version out of PKG-INFO. If we're installed,
+    # this'll let us find our version without pulling in pbr. After all, if
+    # we're installed on a system, we're not in a Git-managed source tree, so
+    # pbr doesn't really buy us anything.
+    __version__ = __canonical_version__ = pkg_resources.get_provider(
+        pkg_resources.Requirement.parse('swift')).version
+except pkg_resources.DistributionNotFound:
+    # No PKG-INFO? We're probably running from a checkout, then. Let pbr do
+    # its thing to figure out a version number.
+    import pbr.version
+    _version_info = pbr.version.VersionInfo('swift')
+    __version__ = _version_info.release_string()
+    __canonical_version__ = _version_info.version_string()
 
 _localedir = os.environ.get('SWIFT_LOCALEDIR')
 _t = gettext.translation('swift', localedir=_localedir, fallback=True)