fix(cmp): discover plugins relative to app path (#13940)

[crenshaw-dev]

According to our docs, CMP discovery was always meant to be relative to the repo root. That was a reasonable design, because discovery for other config management tools (e.g. Helm) is done at the repo root.

But those other tools have multiple outputs: the names of the directories which they can handle. CMP discovery only has a boolean output: "yes, I support that directory" or "no, I do not support that directory."

So, the actual implementation (in contradiction to the docs) answers that question at the app's path instead of the repo root. That's reasonable, because Argo CD needs to know "will this plugin work in this directory?" Since we only get a yes/no, we have to ask the question about a specific directory, instead of about the whole repo.

#13360 made one important fix: instead of just sending the app directory to the plugin, we send the whole repo. That makes sense, because stuff outside the app directory might be important.

But the PR made a mistake: it changed the discovery mechanism to evaluate discovery rules relative to the repo root instead of the app path.

This PR does two things:

1. It returns CMP discovery to be relative to the app path instead of repo root.
2. It corrects the docs.

This PR does not revert the changes from #13360 which makes the whole repo available to the plugin.

Fixes #13940

[crenshaw-dev]

/cherry-pick release-2.7

[crenshaw-dev]

/cherry-pick release-2.6

[leoluz]

LGTM

[codecov]

Codecov Report

Patch coverage: 28.00% and no project coverage change.

Comparison is base (3c6a6e1) 49.56% compared to head (809ca8d) 49.56%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13946   +/-   ##
=======================================
  Coverage   49.56%   49.56%           
=======================================
  Files         256      256           
  Lines       43920    43927    +7     
=======================================
+ Hits        21770    21774    +4     
- Misses      19987    19990    +3     
  Partials     2163     2163           

Impacted Files	Coverage Δ
util/cert/cert.go	76.56% <0.00%> (ø)
util/git/creds.go	50.93% <0.00%> (ø)
util/gpg/gpg.go	66.00% <0.00%> (ø)
util/helm/cmd.go	31.65% <0.00%> (ø)
util/app/discovery/discovery.go	34.78% <25.00%> (ø)
cmpserver/plugin/plugin.go	79.64% <46.15%> (-1.65%)	⬇️

... and 1 file with indirect coverage changes

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[gcp-cherry-pick-bot]

Cherry-pick failed with Merge error cec07ee1ceecdb31ca9c0cd7e350a9b1041de74a into temp-cherry-pick-0b153a-release-2.7

[gcp-cherry-pick-bot]

Cherry-pick failed with Merge error cec07ee1ceecdb31ca9c0cd7e350a9b1041de74a into temp-cherry-pick-0b153a-release-2.6