[ytdl] add 'module-loction' option

[RadicalRingtail]

this is an option to specify a directory to import either the yt-dlp or youtube-dl module from (meant to address this issue, which i encountered today after updating gallery-dl to the latest version: #4729)

this change also renames the ytdl 'module' option to 'module-name' to prevent confusion as to what each of these 2 options do

[mikf]

PYTHONPATH exists.

---

I think this would be a lot simpler by re-using the existing module option and using it with the util.import_file() function, which supports file system paths as well as module names, instead of plain __import__().

diff --git a/gallery_dl/ytdl.py b/gallery_dl/ytdl.py
index 32545e26..4f0303d8 100644
--- a/gallery_dl/ytdl.py
+++ b/gallery_dl/ytdl.py
@@ -21,5 +21,5 @@ def import_module(module_name):
         except (ImportError, SyntaxError):
             return __import__("youtube_dl")
-    return __import__(module_name.replace("-", "_"))
+    return util.import_file(module_name.replace("-", "_"))