Make go one of the builtin filetypes, add the parser and update the related
source and config files. While there, remove Rust from [Groups] in
filetype_extensions.conf because it's already a builtin filetype as well.
The parser itself is stolen from the fishman/ctags repo.
Don't use wildcards to select files to install, nor manually perform
the installation when Automake could do it for us.
Using wildcards makes it hard to know what will really be installed,
and may results in installing files not part of the distribution.
