cli-plugins/manager: minor cleanups and refactoring

[thaJeztah]

cli-plugins/manager: ListPlugins: pass context to error-group

This error-group was added in 89583b9, but
passed a context.TODO because the function didn't have a context as argument.

However, it does get the root-command passed, which holds the context, so
we can pass that.

cli-plugins/manager: rename var that shadowed arg in test

cli-plugins/manager: add test for empty / non-existing plugin dirs

Verify that listPluginCandidates returns an empty result if nothing was
found.

cli-plugins/manager: ListPlugins: return early if no candidates

Skip the other logic, which includes listing all commands provided; if
there's no plugin-candidates, those steps won't be needed.

cli-plugins/manager: getPluginDirs: remove redundant error-return

This function returned an error (if any) from config.Path. However, the
only situation in which an error could be returned was if the given path
to append to config.Dir was outside of the config directory. This can
only happen if the path to append would try to traverse directories (e.g.,
passing ../../cli-plugins).

Given that we're passing a hard-coded value, that would not be the case,
so we can simplify the code to join the path directly, and don't have to
handle errors.

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Oh! Ignore me; It's the context that's nil - interesting; I recall we once had a discussion about that, but Cobra itself was expected to set this automatically 🤔

=== RUN   TestListPluginsIsSorted
--- FAIL: TestListPluginsIsSorted (0.00s)
panic: cannot create context from nil parent [recovered]
	panic: cannot create context from nil parent

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 61.11111% with 7 lines in your changes missing coverage. Please review.

Project coverage is 59.38%. Comparing base (2d74733) to head (091421f).
Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5936      +/-   ##
==========================================
+ Coverage   59.33%   59.38%   +0.04%     
==========================================
  Files         358      358              
  Lines       29783    29776       -7     
==========================================
+ Hits        17672    17681       +9     
+ Misses      11142    11127      -15     
+ Partials      969      968       -1     

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[Benehiko]

maybe we should instead force the tests to set a context?

[thaJeztah]

Yeah, I originally updated the test, but given that this function is exported, I could not 100% exclude the possibility for other code to call this with a command that has not (yet?) has a context set.

Without a context, this produces a panic at runtime, which was a bit risky, so ultimately I chose for making this "best effort", and keep the test as-is to verify that things work even if no context is present on the command.

[Benehiko]

Ah i see, alright then let's get this one merged.