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/convert.go

@@ -46,6 +46,7 @@ to use JSON for the front matter.`,
 					return c.convertContents(metadecoders.JSON)
 				},
 				withc: func(cmd *cobra.Command, r *rootCommand) {
+					cmd.ValidArgsFunction = cobra.NoFileCompletions
 				},
 			},
 			&simpleCommand{
@@ -57,6 +58,7 @@ to use TOML for the front matter.`,
 					return c.convertContents(metadecoders.TOML)
 				},
 				withc: func(cmd *cobra.Command, r *rootCommand) {
+					cmd.ValidArgsFunction = cobra.NoFileCompletions
 				},
 			},
 			&simpleCommand{
@@ -68,6 +70,7 @@ to use YAML for the front matter.`,
 					return c.convertContents(metadecoders.YAML)
 				},
 				withc: func(cmd *cobra.Command, r *rootCommand) {
+					cmd.ValidArgsFunction = cobra.NoFileCompletions
 				},
 			},
 		},
@@ -108,6 +111,7 @@ func (c *convertCommand) Init(cd *simplecobra.Commandeer) error {
 See convert's subcommands toJSON, toTOML and toYAML for more information.`
 
 	cmd.PersistentFlags().StringVarP(&c.outputDir, "output", "o", "", "filesystem path to write files to")
+	_ = cmd.MarkFlagDirname("output")
 	cmd.PersistentFlags().BoolVar(&c.unsafe, "unsafe", false, "enable less safe operations, please backup first")
 
 	cmd.RunE = nil