fix directly testing packages; freeze more meta.yaml data

[msarahan]

This fixes conda build --test so that the actual package provided is the one that gets installed for the test. It no longer tries to go download it from somewhere, based on metadata provided by rendering the recipe in the package.

This hard-codes important information in the rendered meta.yaml file:

• exact pins for build dependencies (even those not explicitly listed - it is whatever is present in the build env)
• absolute path substituted for relative source/path or source/git_url entries (necessary for testing)
• the build string, so that no further rendering is necessary to come up with a similar package spec.

Finally, this adds one option to conda build, --output-folder, which dumps the output package to some folder. This is useful for CI (particularly concourse CI), where output folder affects how artifacts are carried to the next stage.

closes #1534

[msarahan]

CC @groutr - I used some of your code from conda-tools here. Please speak up if any part of that is not OK.

[github-actions]

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