Completion: Hard-code directories where to install completion scripts

Requiring having a shell installed at build-time to get the appropriate
location won't cover the cases like when a system has such a location
set to a non-standard path.

The use of fallbacks together with hardcoded paths also increases the
complexity of this particular feature.

All 3 major shells (Bash, Zsh, Fish) have documented default directories
where they look for completion scripts and we can make use of that.
Additionaly, hard-coding of these paths seems to be the standard among
projects.

Fallout from containers#840

containers#1055

completion/meson.build

@@ -1,16 +1,6 @@
-if bash_completion.found()
-  bash_comp_dir = bash_completion.get_pkgconfig_variable('completionsdir')
-else
-  bash_comp_dir = get_option('datadir') / 'bash-completion' / 'completions'
-  message('bash-completion not found: using', get_option('prefix') / bash_comp_dir, 'as a falback install directory')
-endif
-
-if fish.found()
-  fish_comp_dir = fish.get_pkgconfig_variable('completionsdir')
-else
-  fish_comp_dir = get_option('datadir') / 'fish' / 'completions'
-  message('fish not found: using', get_option('prefix') / fish_comp_dir, 'as a fallback install directory')
-endif
+bash_comp_dir = get_option('datadir') / 'bash-completion' / 'completions'
+zsh_comp_dir = get_option('datadir') / 'zsh' / 'site-functions'
+fish_comp_dir = get_option('datadir') / 'fish' / 'vendor_completions.d'
 
 completion_bash = custom_target('bash-completion',
   capture: true,
@@ -23,7 +13,7 @@ completion_zsh = custom_target('zsh-completion',
   capture: true,
   command: [toolbox, 'completion', 'zsh'],
   install: true,
-  install_dir: get_option('datadir') / 'zsh' / 'site-functions',
+  install_dir: zsh_comp_dir,
   output: '_toolbox')
 
 completion_fish = custom_target('fish-completion',

meson.build

@@ -18,9 +18,6 @@ go_md2man = find_program('go-md2man')
 shellcheck = find_program('shellcheck', required: false)
 skopeo = find_program('skopeo', required: false)
 
-bash_completion = dependency('bash-completion', required: false)
-fish = dependency('fish', required: false)
-
 migration_path_for_coreos_toolbox = get_option('migration_path_for_coreos_toolbox')
 profiledir = get_option('profile_dir')