Adds the new linter and configures it to prevent direct error
comparisions and type assertions in favor of errors.Is and errors.As.
All current violations caught by the linter have been updated
accordingly.
This commit updates all `err == io.EOF` comparisons to use
`errors.Is(err, io.EOF)`. This is necessary when the error may have been
wrapped and fixes at least one current breakage (in `tsh request ls`).
`golang.org/x/tools/refactor/eg` was very handy for this, I used the
following template:
```go
package teleport
import (
"errors"
"io"
)
func before(err error) bool { return err == io.EOF }
func after(err error) bool { return errors.Is(err, io.EOF) }
```
* Vendor slack plugin and supporting libraries
* Fix up plugin integration tests (wip)
* Run GCI on vendored code
* Use newtype instead of type alias
golangci-lint currently panics on this,
"skip-files" et al don't help, as it is a linter panic, not an error
See https://github.com/dominikh/go-tools/commit/d717045480130461e1e777ead7f2981621f0d9f0
* Remove long-runing plugins tests from difftest
* Move access plugin tests to unit-tests-integrations
rosstimothy