Comprehend Go1.4 import comment constraints

[sdboyer]

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Go 1.4 introduced import comment constraints that allow the source of a package to designate the import path at which a package must be placed in order for it to compile.

We need to read these comments and at least note their presence - it doesn't matter quite yet, but once we support aliasing, we'll need to constraint that the presence of such a comment disallows incompatible aliasing.

[fabulous-gopher]

This issue was moved to golang/dep#447