Fix crash when trying to overwrite a package

Before minetest#11646, core.copy_dir would overwrite the target if it exists. Adding core.delete_dir restores the exact same behaviour

Fixes minetest#12303

builtin/mainmenu/pkgmgr.lua

@@ -608,11 +608,10 @@ function pkgmgr.install_dir(type, path, basename, targetpath)
  end
 
  local from = basefolder and basefolder.path or path
- if targetpath then
- core.delete_dir(targetpath)
- else
+ if not targetpath then
  targetpath = core.get_texturepath() .. DIR_DELIM .. basename
  end
+ core.delete_dir(targetpath)
  if not core.copy_dir(from, targetpath, false) then
  return nil,
  fgettext("Failed to install $1 to $2", basename, targetpath)
@@ -690,6 +689,7 @@ function pkgmgr.install_dir(type, path, basename, targetpath)
  end
 
  -- Copy it
+ core.delete_dir(targetpath)
  if not core.copy_dir(basefolder.path, targetpath, false) then
  return nil,
  fgettext("Failed to install $1 to $2", basename, targetpath)