Prevent empty-block on range over channel

[avorima]

An empty loop over channels is a common pattern:
https://sourcegraph.com/search?q=context%3Aglobal+lang%3AGo+for%5C+range%5C+.*%5C+%5C%7B%5Cs*%5C%7D&patternType=regexp&sm=1&groupBy=repo
https://sourcegraph.com/search?q=context%3Aglobal+lang%3AGo+for%5C+range%5C+.*%5C+%5C%7B%5Cs*%5C%7D+repo%3Agithub.com%2Fgolang%2Fgo&patternType=regexp&sm=1&groupBy=repo

Since the given AST doesn't really provide much type info I had to resort to finding the declaration/definition of the object to figure out whether for range was actually looping over a channel.
This doesn't feel very reliable, but it works for the test data. I'll be happy to extend the checks if an issue gets reported.

I tried re-adding file.Pkg.TypeCheck() from #415, but didn't notice any difference in the AST.

Closes #386