vim-plugins: prefix all plugin names with 'vimplugin-'

Currently some plugins use "vim-" prefix, some use "-vim" suffix and
some have no prefix or suffix at all.

I chose "vimplugin-" prefix to match the attribute path: vimPlugins.*
vim-plugins: buildVimPlugin: drop needless 'path' argument

Derive 'path' from the plugin name instead.
vim-plugins: update source hash for command-t

And use fetchzip instead of fetchurl towards github.com to not let this
happen again. (fetchzip checks the hash on the *extracted* archive, so
changes in compression algo etc. doesn't change the hash.)
vim-plugins: use lower case attr and pkg names

(According to the nixpkgs manual.)

Keep old attrnames around for backward compatibility.