Basic dependency checking. setup() has two new optional arguments

requires and provides. requires is a sequence of strings, of the
form 'packagename-version'. The dependency checking so far merely
does an '__import__(packagename)' and checks for packagename.__version__
You can also leave off the version, and any version of the package
will be installed.
There's a special case for the package 'python' - sys.version_info
is used, so
requires= ( 'python-2.3', )
just works.

Provides is of the same format as requires - but if it's not supplied,
a provides is generated by adding the version to each entry in packages,
or modules if packages isn't there.
Provides is currently only used in the PKG-INFO file. Shortly, PyPI
will grow the ability to accept these lines, and register will be
updated to send them.

There's a new command 'checkdep' command that runs these checks.
For this version, only greater-than-or-equal checking is done. We'll
add the ability to specify an optional operator later.

Lib/distutils/command/checkdep.py

@@ -0,0 +1,70 @@
+"""distutils.command.x
+
+Implements the Distutils 'x' command.
+"""
+
+# created 2000/mm/dd, John Doe
+
+__revision__ = "$Id$"
+
+from distutils.core import Command
+
+class DependencyFailure(Exception): pass
+
+class VersionTooOld(DependencyFailure): pass
+
+class VersionNotKnown(DependencyFailure): pass
+
+class checkdep (Command):
+
+    # Brief (40-50 characters) description of the command
+    description = "check package dependencies"
+
+    # List of option tuples: long name, short name (None if no short
+    # name), and help string.
+    # Later on, we might have auto-fetch and the like here. Feel free.
+    user_options = []
+
+    def initialize_options (self):
+        self.debug = None
+
+    # initialize_options()
+
+
+    def finalize_options (self):
+        pass
+    # finalize_options()
+
+
+    def run (self):
+        from distutils.version import LooseVersion
+        failed = []
+        for pkg, ver in self.distribution.metadata.requires:
+            if pkg == 'python':
+                if ver is not None:
+                    # Special case the 'python' package
+                    import sys
+                    thisver = LooseVersion('%d.%d.%d'%sys.version_info[:3])
+                    if thisver < ver:
+                        failed.append(((pkg,ver), VersionTooOld(thisver)))
+                continue
+            # Kinda hacky - we should do more here
+            try:
+                mod = __import__(pkg)
+            except Exception, e:
+                failed.append(((pkg,ver), e))
+                continue
+            if ver is not None:
+                if hasattr(mod, '__version__'):
+                    thisver = LooseVersion(mod.__version__)
+                    if thisver < ver:
+                        failed.append(((pkg,ver), VersionTooOld(thisver)))
+                else:
+                    failed.append(((pkg,ver), VersionNotKnown()))
+
+        if failed:
+            raise DependencyFailure, failed
+
+    # run()
+
+# class x