cmp/cmpopts: use errors.Is with ≥go1.13 in compareErrors

[tklauser]

Use the standard definition of errors.Is to implement compareErrors with
≥go1.13. Retain the implementation using golang.org/x/xerrors for versions <go1.13.

This will allow packages using newer Go versions and already relying on
the errors package to get rid of the transitive dependency on
golang.org/x/xerrors.

[tklauser]

I see that previously #225 and #243 attempted to get rid of the golang.org/x/xerrors as well. However it looks like these weren't accepted based on the fact that they broke cmp for Go versions older than 1.13, while this PR retains compatibility for older versions.

[dsnet]

What's the effect of this PR on the go.mod file?

[tklauser]

What's the effect of this PR on the go.mod file?

As far as I can tell there will be no effect on the go.mod file because cmp/cmpopts/util_test.go still uses xerrors unconditionally. Running go mod tidy doesn't change go.mod or go.sum.

However, a package importing cmp which uses go1 .13 or newer in go.mod should no longer see an indirect dependency on xerrors.

[dsnet]

The presence of xerrors in the go.mod file is unfortunate especially since older versions of Go (which is why the dependency even needs to exist) don't use Go modules.

Anyways, I agree this change in a step in the direction we want to go.