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Warn user if unsupported options are supplied, and cancel unless
'depends -i' (aka '--ignore-extra-args') was used.

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branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4040880

all requirements aren't met.  (Also, check planned install location for
the dependencies, as well as checking sys.path.)  Also:

* Allow 'Feature()' objects to include 'Require()' objects, so that
  dependencies can be optional

* 'Require()' objects can set a homepage, whose URL will be displayed by
  the 'depends' command if the dependency needs to be installed.

* Misc. fixes/refactoring of version validation to properly handle
  "unknown" versions, and to decouple version fetching from version
  checking.

* Updated TODO to remove various completed items.

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branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4040876

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branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4040869