bug: cannot use plugins that create symlinks in 'installs'

[yacchi]

Describe the Bug

When installing a plugin that creates a symbolic link under the installs directory, asdf fails to recognize or use it.
This behavior differs from the previous Bash-based system, where symlink-based plugins worked without issue.

The current implementation appears to ignore symlinks by checking for directories only, rendering any plugin that relies on creating a symlink in installs unusable.

Steps to Reproduce

1. Install a plugin that relies on creating a symlink (e.g., go-sdk).
2. Immediately after installing, run asdf list and confirm that it shows No compatible versions installed.
3. Install any version for the plugin.
4. Run asdf list again and confirm that the installed version is not listed.
5. Check the plugin’s installs directory and verify that a symlink for the installed version exists.

Since the version is not recognized, it is unclear whether the tool can be used.

Expected Behaviour

Plugins that create symlinks in the installs directory should be recognized and listed by asdf just like directory-based plugins. Any installed versions should appear in asdf list and be fully usable.

Actual Behaviour

Any plugin creating symlinks under the installs directory is not recognized by asdf. Though the symlinks are created, versions do not appear in the asdf list, making the plugin unusable.

Environment

OS:
Darwin fujie-MacBookPro.local 23.5.0 Darwin Kernel Version 23.5.0: Wed May  1 20:09:52 PDT 2024; root:xnu-10063.121.3~5/RELEASE_X86_64 x86_64

SHELL:
fish, version 3.5.0

BASH VERSION:
5.1.8(1)-release

ASDF VERSION:
0.16.2

asdf plugins affected (if relevant)

go-sdk