crypto/tls/internal/fips140tls: use crypto/fips140

crypto/tls/internal/fips140tls: use crypto/fips140

There is no need for fips140tls to depend on an internal package, it
can use crypto/fips140 directly.

Both approaches are equivalent, but using crypto/fips140 makes us
exercise a public API and sets precedence.

Change-Id: I668e80ee62b711bc60821cee3a54232a33295ee1
Reviewed-on: https://go-review.googlesource.com/c/go/+/642035
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>