allow channel auth when checking if package is built

[msarahan]

fixes #3127

@teake - please test. Please submit a PR with a test, too, if you can. I don't have time for more than just throwing up what @kalefranz proposed in #3127

[teake]

Adding a test is a bit tricky, as I'll have to mock a simple HTTP file server with authentication. I'm afraid I'll have to pass on that one.

I did test it though against my original use case, and it seems to work after a few fixes. Not sure how to get that into this PR, but I've created a PR-to-PR: msarahan#11.

[teake]

I forgot to mention that update_index gives a warning after this change:

WARNING:conda_build.index:The update_index function has changed to index all subdirs at once.  You're pointing it at a single subdir.  Please update your code to point it at the channel root, rather than a subdir.

It didn't before because it didn't get called as it was (wrongly?) indented differently.

[msarahan]

Thanks @teake. I'll fix the usage of update_index that's showing the warning.

[msarahan]

Test fails are some unrelated screwup. I have fixed the update_index warning elsewhere. Merging. Thanks @teake

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