cmd/go/internal/load: always use DefaultExecName to determine binary name

It should produce equivalent results to split on the import path of the
package rather than its directory, in GOPATH mode. That means the common
code in DefaultExecName can be used.

We're in the middle of Go 1.12 cycle, so now is a good time to make it
happen for Go 1.13.

Modify isVersionElement to accept path elements like "v2", "v3", "v10",
rather than "/v2", "/v3", "/v10". Then use it in DefaultExecName instead
of the ad-hoc isVersion function. There is no change in behavior.

Add tests for DefaultExecName and isVersionElement.

Updates #26869

Change-Id: Ic6da2c92587459aa2b327385e994b72a6e183092
Reviewed-on: https://go-review.googlesource.com/c/go/+/168678


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>