cmd/dist: include "go1.x-" in devel go version strings

This way, "go version" will report the "base version" or major version
that the tool corresponds to. This is the same version number that is
matched against build tags such as "go1.14" or "!go1.16".

Obtaining this version being built is non-trivial, since we can't just
import a package or query git. The added comments document the chosen
mechanism, based on a regular expression. It was chosen over AST parsing
as it would add a significant amount of code without much gain, given
how simple the goversion.go file is.

For #41116.

Change-Id: I653ae935e27c13267f23898f89c84020dcd6e194
Reviewed-on: https://go-review.googlesource.com/c/go/+/264938


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>