stack: use builtin completion command (#6230)

plugins/stack/stack.plugin.zsh

-function _stack_commands() {
-    local ret=1 state
-    _arguments ':subcommand:->subcommand' && ret=0
-
-    case $state in
-      subcommand)
-        subcommands=(
-          "build:Build the project(s) in this directory/configuration"
-          "install:Build executables and install to a user path"
-          "test:Build and test the project(s) in this directory/configuration"
-          "bench:Build and benchmark the project(s) in this directory/configuration"
-          "haddock:Generate haddocks for the project(s) in this directory/configuration"
-          "new:Create a brand new project"
-          "init:Initialize a stack project based on one or more stack packages"
-          "solver:Use a dependency solver to try and determine missing extra-deps"
-          "setup:Get the appropriate ghc for your project"
-          "path:Print out handy path information"
-          "unpack:Unpack one or more packages locally"
-          "update:Update the package index"
-          "upgrade:Upgrade to the latest stack (experimental)"
-          "upload:Upload a package to Hackage"
-          "dot:Visualize your project's dependency graph using Graphviz dot"
-          "exec:Execute a command"
-          "ghc:Run ghc"
-          "ghci:Run ghci in the context of project(s)"
-          "ide:Run ide-backend-client with the correct arguments"
-          "runghc:Run runghc"
-          "clean:Clean the local packages"
-          "docker:Subcommands specific to Docker use"
-        )
-        _describe -t subcommands 'stack subcommands' subcommands && ret=0
-    esac
-
-    return ret
-}
-
-compdef _stack_commands stack
+autoload -U +X bashcompinit && bashcompinit
+source <(stack --bash-completion-script stack)