all: re-introduce go.mod and go.sum files

Now that tools' dependencies have been cleaned up in golang.org/cl/160837, lint
doesn't have any transitive dependencies on modules that import it using the
wrong path. It is now safe to reintroduce a go.mod file to this repo. I've
checked using dmitshur's instructions in
https://github.com/golang/lint/issues/436#issuecomment-469048301 that lint only
appears with its canonical module path in the build list.

Updates golang/lint#436

Change-Id: I6343aa103408b20562e17ea019602b159b899fc6
Reviewed-on: https://go-review.googlesource.com/c/lint/+/166278
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

go.mod

+module golang.org/x/lint
+
+require golang.org/x/tools v0.0.0-20190311212946-11955173bddd

go.sum

+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/tools v0.0.0-20190311212946-11955173bddd h1:/e+gpKk9r3dJobndpTytxS2gOy6m5uvpg+ISQoEcusQ=
+golang.org/x/tools v0.0.0-20190311212946-11955173bddd/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=