Add support for listing and installing dev branches

[liggitt]

Includes ability to list/install from dev branches:

gvm listall -a

gvm gos (available)

   ...
   dev.boringcrypto
   dev.cc
   dev.cmdgo
   dev.debug
   dev.fuzz
   dev.garbage
   dev.gcfe
   dev.go2go
   dev.inline
   dev.link
   dev.power64
   dev.regabi
   dev.ssa
   dev.tls
   dev.typealias
   dev.typeparams
   dev.types
   dev.boringcrypto.go1.8
   dev.boringcrypto.go1.9
   dev.boringcrypto.go1.10
   dev.boringcrypto.go1.11
   dev.boringcrypto.go1.12
   dev.boringcrypto.go1.13
   dev.boringcrypto.go1.14
   dev.boringcrypto.go1.15
   dev.boringcrypto.go1.16

gvm install dev.boringcrypto.go1.16 --name=go1.16b
Installing dev.boringcrypto.go1.16 as go1.16b...
 * Compiling...
go1.16b successfully installed!

And since branches can change over time, also allows force-updating source in install with --update-source so the following works:

gvm uninstall go1.16b && gvm install dev.boringcrypto.go1.16 --update-source --name=go1.16b

[liggitt]

Hi @BenKnigge, I found it useful to be able to install and manage go builds from branches as well as tags, and the change was pretty minimal. Thought I'd share it here in case it would be useful for other people as well.