cmd/api: rename api.go to main_test.go

cmd/api: rename api.go to main_test.go

This makes cmd/api no longer an importable package.
In CL 453258 I forgot that there was no direct prohibition
on importing packages from cmd - we just rely on the
fact that cmd/* is all package main and everything else
is cmd/internal.

Fixes #62069.

Change-Id: Ifed738d333b40663f85eca8f83025fcea5df89a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/520038
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>