completion: Improve existing argument completions, add many more

Do not offer filenames to arguments not taking one, complete arguments
of options taking resource kinds, directory names, --logLevel, release
--step, config and new --format.

As an internal refactoring, use higher level functions to set flag
completions.  SetAnnotation works, but is more verbose than
alternatives, and uses bash specific wording.

While at it, move setting completions next to flag definitions
consistently.

Remove superfluous --destination completer setting, which is already
set elsewhere.

commands/config.go

@@ -28,6 +28,7 @@ import (
 	"github.com/gohugoio/hugo/modules"
 	"github.com/gohugoio/hugo/parser"
 	"github.com/gohugoio/hugo/parser/metadecoders"
+	"github.com/spf13/cobra"
 )
 
 // newConfigCommand creates a new config command and its subcommands.
@@ -112,7 +113,9 @@ func (c *configCommand) Init(cd *simplecobra.Commandeer) error {
 	cmd.Short = "Print the site configuration"
 	cmd.Long = `Print the site configuration, both default and custom settings.`
 	cmd.Flags().StringVar(&c.format, "format", "toml", "preferred file format (toml, yaml or json)")
+	_ = cmd.RegisterFlagCompletionFunc("format", cobra.FixedCompletions([]string{"toml", "yaml", "json"}, cobra.ShellCompDirectiveNoFileComp))
 	cmd.Flags().StringVar(&c.lang, "lang", "", "the language to display config for. Defaults to the first language defined.")
+	_ = cmd.RegisterFlagCompletionFunc("lang", cobra.NoFileCompletions)
 	applyLocalFlagsBuildConfig(cmd, c.r)
 
 	return nil
@@ -223,6 +226,7 @@ func (c *configMountsCommand) Init(cd *simplecobra.Commandeer) error {
 	c.r = cd.Root.Command.(*rootCommand)
 	cmd := cd.CobraCommand
 	cmd.Short = "Print the configured file mounts"
+	cmd.ValidArgsFunction = cobra.NoFileCompletions
 	applyLocalFlagsBuildConfig(cmd, c.r)
 	return nil
 }