[MERGE] FileCollection

[Robert Collins]

This adds a little utility class to support file discovery on unlistable
transports.

While I think removing support for unlistable readonly transports is a
reasonable thing to do, I don't want to conflate that change with the
introduction of undiscoverable files which my many-file index work does.

So this is just a little tool to allow access over http to my test
repositories - the indices they build get allocated to a filecollection,
and removed on gc/pack operations. As we should never have more than 100
or so files (using a logarithmic backoff this is a huge number of
entries) I think this is reasonable in the interim.

-Rob
-- 
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