[28.x backport] cli-plugins/manager: ignore broken symlinks

[thaJeztah]

• backport cli-plugins/manager: ignore broken symlinks #5922

Before this patch, a broken symlink would print a warning;

docker info > /dev/null
WARNING: Plugin "/Users/thajeztah/.docker/cli-plugins/docker-feedback" is not valid: failed to fetch metadata: fork/exec /Users/thajeztah/.docker/cli-plugins/docker-feedback: no such file or directory

After this patch, such symlinks are ignored:

docker info > /dev/null

With debug enabled, we don't ignore the faulty plugin, which will make the warning shown on docker info;

mkdir -p ~/.docker/cli-plugins
ln -s nosuchplugin ~/.docker/cli-plugins/docker-brokenplugin
docker --debug info
Client:
 Version:    29.0.0-dev
 Context:    default
 Debug Mode: true
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.25.0
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
WARNING: Plugin "/Users/thajeztah/.docker/cli-plugins/docker-brokenplugin" is not valid: failed to fetch metadata: fork/exec /Users/thajeztah/.docker/cli-plugins/docker-brokenplugin: no such file or directory

# ...

We should als consider passing a "seen" map to de-duplicate entries. Entries can be either a direct symlink or in a symlinked path (for which we can filepath.EvalSymlinks). We need to benchmark the overhead of resolving the symlink vs possibly calling the plugin (to get their metadata) further down the line.

(cherry picked from commit 9b2f831)

- What I did

- How I did it

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- Human readable description for the release notes

Don't print warnings in `docker info` for broken symlinks in CLI-plugin directories

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