This is a result of running `go mod tidy` using go 1.13.6.
In my devel toolchain (vim-go -> gopls -> go 1.13), go apparently
wants to add this line to go.mod, but since the modules are read-only,
it fails, which leads to gopls failing, too.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>