all: add basic version skew detection

[josharian]

cc @thepudds @mvdan

[thepudds]

Conceptually, this seems reasonable and useful, and hopefully helps with modules support introducing greater chances of version skew with modules.

The initial platforms are passing in travis...

(I did not look carefully at each line of code at this point).

[dvyukov]

Long term it would be good to get rid of the archive as it makes integration with other systems harder.
I would inject the version into the generated main function and pass back to dep.Main. It can even make code simpler (no need to read/write files).

[josharian]

Long term it would be good to get rid of the archive as it makes integration with other systems harder.

Agreed. I don't quite know how to do this. Opened #277 for further discussion of this topic.

I would inject the version into the generated main function and pass back to dep.Main. It can even make code simpler (no need to read/write files).

That requires also serializing and passing it along to the runner. But that assumes that serialized communication is working correctly, which it might not be in the presence of version skew.