cmd/go: make mod init disallow invalid major version suffixes
This CL reuses the SplitPathVersion function from the module package to
detect invalid major version suffixes and return a relevant error
message along with a suggested fix.
Fixes #44052
Fixes #46085
Change-Id: I6c06f31a134e864a1d9b6e00c048ca1c59b4365e
Reviewed-on: https://go-review.googlesource.com/c/go/+/288712 Reviewed-by: Jay Conrod <jayconrod@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>