Go 1.13 - Use `go/token.IsIdentifier` Function for `transform.ValidIdentifierKeys`

[Rican7]

Go 1.13 is adding a new IsIdentifier function to the go/token package in the standard library:

• golang/go#30064
• https://go-review.googlesource.com/c/go/+/169018/
• golang/go@60a8dbf3b4e10627b9f5c3a0a0bf4462247270c8

We could use that for either the logic in transform.ValidIdentifierKeys or at least the test, if we don't want to actually import go/token for the "distributed" package.