Add metapackages: golang-latest, golang-latest-default

[Piotr Henryk Dabrowski]

Ubuntu provides an outdated golang 1.10 by default.

And it is very inconvenient to use the 'golang-1.12' package, because:

1) User must specify the exact version.
A 'golang-latest' metapackage would be very useful here.

2) 'golang-1.12' does not provide go/gofmt binaries anywhere in $PATH which is very confusing.
Metapackages like 'golang-latest-default', 'golang-default', 'golang-1.11-default', 'golang-1.12-default' should exist to allow simple setting of alternatives for /usr/bin/go and /usr/bin/gofmt with just one apt install command.

To summarize:
One should be able to setup latest Go that works out-of-the-box with one simple command:
$ sudo apt install golang-latest-default