refactor(external-open): extract external-open-lib for shared helpers

Same shared-helpers split-pattern that ai-vterm/vterm-config use
through cj-window-toggle-lib and that calendar-sync uses through
cj-org-text-lib.  Pull the two pure dispatch helpers out of the
external-open feature module into a sibling library so consumers
that only need the dispatch don't have to require the whole feature.

New `modules/external-open-lib.el' carries:
- `cj/external-open-command'
- `cj/external-open-launcher-p'

`modules/external-open.el' stays as the feature module: the
`default-open-extensions' defcustom, the `find-file' advice
(`cj/find-file-auto'), and the interactive commands (`cj/xdg-open',
`cj/open-this-file-with').  It now requires external-open-lib for
the dispatch helpers.

Migrate consumers:
- system-utils.el used to require `external-open' for
  `cj/external-open-launcher-p' alone -- now requires
  `external-open-lib' directly.
- dirvish-config.el calls `cj/external-open-command' from
  `cj/dirvish-open-file-manager-here' -- add an explicit
  `(require \='external-open-lib)'.

Test files renamed to match the system-lib naming pattern
(test-<library>-<feature>.el):
- test-external-open-command.el -> test-external-open-lib-command.el
- test-external-open-launcher-p.el -> test-external-open-lib-launcher-p.el

No behavior change.

1 files changed, 1 insertions, 1 deletions

diff --git a/modules/system-utils.el b/modules/system-utils.el
index 008a5396..43200403 100644
--- a/modules/system-utils.el
+++ b/modules/system-utils.el
@@ -24,7 +24,7 @@
 ;;; Code:
 
 (require 'system-lib)
-(require 'external-open)
+(require 'external-open-lib)
 
 (declare-function dired-get-file-for-visit "dired" ())
 (declare-function dired-file-name-at-point "dired" ())