This lets us simplify the code considerably. For example, unquoting the
tag is no longer necessary, and we can get the field name with a single
method call.

While at it, fix a typechecking error in testdata/structtag.go, which
hadn't been caught since vet still skips past go/types errors in most
cases.

Using go/types will also let us expand the structtag check more easily
if we want to, for example to allow it to check for duplicates in
embedded fields.

Finally, update one of the test cases to check for regressions when we
output invalid tag strings. We also checked that these two changes to
testdata/structtag.go didn't fail with the old structtag check.

For #25593.

Change-Id: Iea4906d0f30a67f36b28c21d8aa96251aae653f5
Reviewed-on: https://go-review.googlesource.com/115676
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Rob Pike <r@golang.org>