extension_control_path: ignore empty elements and deduplicate directories

Hi,

While reviewing patch [1]/messages/by-id/CABkiuWpGiLzDg4D8zRBJ6DHnwYgHqkFxz2gDstXgeOdzh6bdZQ@mail.gmail.com, I noticed two issues with extension path population.

1. Empty path elements are silently interpreted as "/extension"

Say a user sets extension_control_path = '$system:', it will be interpreted as two paths: $system and an empty string, and the empty string will be expanded to "/extension". Since "/extension" is an absolute path, it will be searched for extensions. If the host machine doesn't have "/extension", the path will be silently ignored; but if the directory happens to exist, it will be searched for extensions, which doesn't sound desirable, because the user might not be aware of that at all.

Adding a trailing colon is an easy mistake to make, but silently searching "/extension" is unintended behavior. So I think we should silently discard empty path elements. An entirely empty string ('') is interpreted as $system as documented, and that behavior should be retained.

2. It may read the same directory more than once

Say a user sets extension_control_path = '$system:$system', that's an obvious duplicate path, but the current implementation will scan $system twice when listing available extensions, and only deduplicate extension names later.

There is no correctness problem here, but deduplicating search paths may avoid unnecessary directory reads. I think this is a relatively low-priority but useful optimization.

I split the fixes into two commits, so that committers can handle them separately.

I added DEBUG3 logging to print the paths to search, mainly so the tests can verify them. It also allows users to view the expanded search paths.

[1]: /messages/by-id/CABkiuWpGiLzDg4D8zRBJ6DHnwYgHqkFxz2gDstXgeOdzh6bdZQ@mail.gmail.com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/