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
They will be deleted from their current homes once this has landed.
Changes made to import paths to make the code compile, and to find
errchk in the right place in cmd/vet's Makefile.
TODO in a later CL: tidy up vet.
R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/9495043
