This removes much of the AST logic out of main.go,
and makes it easier to build custom vet binaries
The trade-off in this change is for flexibility.
There's very little change in the per-check files,
a lot less code in main.go (specifically the AST walking
logic has shrunk), and it makes it much easier to build
custom vet binaries simply by dropping new source files
in the directory.
LGTM=josharian, r
R=r, josharian, kamil.kisiel
CC=golang-codereviews
https://golang.org/cl/83400043
f895b43688