Comprehend Go1.4 import comment constraints

[fabulous-gopher]

From @sdboyer on June 18, 2016 2:44

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.

Copied from original issue: sdboyer/gps#38

[sdboyer]

closing this in favor of the more detailed #902