0compile: pretty_print_plan fails with KeyError for non-OS selections

For my nodejs source feed, it <requires> gcc_c++ with arch="Linux",
since I don't have a feed for gcc on OSX (you just have to make sure
you have the xcode tools).

On a mac, I get:

$ 0install run http://0install.net/2006/interfaces/0compile.xml
autocompile http://gfxmonk.net/dist/0install/node.js.xml
=============== http://gfxmonk.net/dist/0install/node.js.xml ===============

Selecting versions for node.js...
Selection done.

Plan:

- node.js: Compile 0.10.37 (sha1new=1f4acb3bc4749c7aaafe9757f09c341386a816dd)
Traceback (most recent call last):
  File "/Users/test/.cache/0install.net/implementations/sha256new_V3EKZQ5AWLF64GJ6QZ5AQLZEWV7ZWONFZ6JNC4DE2RUIBXEBJ2CQ/0compile",
line 78, in <module>
    matches[0](args[1:])
  File "/Users/test/.cache/0install.net/implementations/sha256new_V3EKZQ5AWLF64GJ6QZ5AQLZEWV7ZWONFZ6JNC4DE2RUIBXEBJ2CQ/autocompile.py",
line 586, in do_autocompile
    compiler.build()
  File "/Users/test/.cache/0install.net/implementations/sha256new_V3EKZQ5AWLF64GJ6QZ5AQLZEWV7ZWONFZ6JNC4DE2RUIBXEBJ2CQ/autocompile.py",
line 363, in build
    tasks.wait_for_blocker(self.recursive_build(self.iface_uri))
  File "/Users/test/.cache/0install.net/implementations/sha256new_DQ5LNKJ5NVYFL5UQWV7S2NLHWI4TUXDZCZZI65L5YKHEHNVNVSLA/zeroinstall/support/tasks.py",
line 501, in wait_for_blocker
    check(blocker)
  File "/Users/test/.cache/0install.net/implementations/sha256new_DQ5LNKJ5NVYFL5UQWV7S2NLHWI4TUXDZCZZI65L5YKHEHNVNVSLA/zeroinstall/support/tasks.py",
line 175, in check
    support.raise_with_traceback(ex[0], ex[1])
  File "/Users/test/.cache/0install.net/implementations/sha256new_DQ5LNKJ5NVYFL5UQWV7S2NLHWI4TUXDZCZZI65L5YKHEHNVNVSLA/zeroinstall/support/__init__.py",
line 105, in raise_with_traceback
    exec("raise ex, None, tb", {'ex': ex, 'tb': tb})    # Python 2
  File "/Users/test/.cache/0install.net/implementations/sha256new_DQ5LNKJ5NVYFL5UQWV7S2NLHWI4TUXDZCZZI65L5YKHEHNVNVSLA/zeroinstall/support/tasks.py",
line 374, in _resume
    new_blockers = next(self.iterator)
  File "/Users/test/.cache/0install.net/implementations/sha256new_V3EKZQ5AWLF64GJ6QZ5AQLZEWV7ZWONFZ6JNC4DE2RUIBXEBJ2CQ/autocompile.py",
line 304, in recursive_build
    self.pretty_print_plan(d.solver, r.interface_uri)
  File "/Users/test/.cache/0install.net/implementations/sha256new_V3EKZQ5AWLF64GJ6QZ5AQLZEWV7ZWONFZ6JNC4DE2RUIBXEBJ2CQ/autocompile.py",
line 154, in pretty_print_plan
    self.pretty_print_plan(solver, x.interface, indent)
  File "/Users/test/.cache/0install.net/implementations/sha256new_V3EKZQ5AWLF64GJ6QZ5AQLZEWV7ZWONFZ6JNC4DE2RUIBXEBJ2CQ/autocompile.py",
line 135, in pretty_print_plan
    impl = solver.selections[iface]
  File "/Users/test/.cache/0install.net/implementations/sha256new_DQ5LNKJ5NVYFL5UQWV7S2NLHWI4TUXDZCZZI65L5YKHEHNVNVSLA/zeroinstall/injector/selections.py",
line 454, in __getitem__
    sel = self.selections[key.uri]
KeyError: u'http://gfxmonk.net/dist/0install/gcc-c.xml'


Ignoring the KeyError in pretty_print_plan makes it work - so it's
just an issue with the diagnostics, the rest of 0compile handles this
case fine.

I'm not sure how best to fix this though - should we ignore the
KeyError entirely, or do a check for some property on the
`impl.requires` element to determine whether it _should_ be present?

Cheers,
 - Tim.

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