params/metrics: do not fail during dir expansion

[skshetry]

Closes #10018.

We have to be careful when expanding paths. This PR fall backs to assume it
as a file when either of isdir and find raises error due to some mishaps.
The consequence of this assumption happens when we try to read the file.
In that case, if the directory exists, and we happen to try to open it
as a file, dvc will raise IsADirectoryError, but that is handled correctly
and won't fail catastrophically like this did.

This is what I get now:

$ dvc metrics show --json
DVC failed to load some metrics for following revisions: ''.
{
  "": {
    "data": {
      "results/train/metrics.json": {
        "error": {
          "type": "DataIndexDirError",
          "msg": "failed to load directory ('results', 'train')"
        }
      },
      "results/evaluate/metrics.json": {
        "data": {
          "dice_multi": 0.8789161007343483
        }
      }
    }
  }
}

The error message is not great, but at least it won't fail.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.71%. Comparing base (5379aa2) to head (02453e0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10375   +/-   ##
=======================================
  Coverage   90.70%   90.71%           
=======================================
  Files         501      501           
  Lines       38837    38865   +28     
  Branches     5616     5618    +2     
=======================================
+ Hits        35229    35257   +28     
  Misses       2964     2964           
  Partials      644      644           

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[shcheklein]

thanks, @skshetry !