`lib list` should not report empty folders under `<directories.user>/libraries` as a `is_legacy` library

[kittaakos]

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Bug Report

Empty folders under <directories.user>/libraries show up as is_legacy: true libraries. If the folder is empty (or contains non-sketch/library files), maybe it should be ignored.

Current behavior

Steps to reproduce:

% rm -rf ~/Documents/Arduino && rm -rf ~/Library/Arduino15
% ./arduino-cli version --format json                     
{
  "Application": "arduino-cli",
  "VersionString": "0.22.0",
  "Commit": "65f662a7",
  "Status": "",
  "Date": "2022-05-06T11:09:35Z"
}
% ./arduino-cli lib list --format json                    
[]
% mkdir -p ~/Documents/Arduino/libraries/nothing_here     
% ./arduino-cli lib list --format json               
[
  {
    "library": {
      "name": "nothing_here",
      "architectures": [
        "*"
      ],
      "install_dir": "/Users/a.kitta/Documents/Arduino/libraries/nothing_here",
      "source_dir": "/Users/a.kitta/Documents/Arduino/libraries/nothing_here",
      "is_legacy": true,
      "location": 1
    }
  }
]
%

Expected behavior

The empty folder is ignored by the lib list command:

% rm -rf ~/Documents/Arduino && rm -rf ~/Library/Arduino15
% ./arduino-cli version --format json                     
{
  "Application": "arduino-cli",
  "VersionString": "0.22.0",
  "Commit": "65f662a7",
  "Status": "",
  "Date": "2022-05-06T11:09:35Z"
}
% ./arduino-cli lib list --format json                    
[]
% mkdir -p ~/Documents/Arduino/libraries/nothing_here     
% ./arduino-cli lib list --format json               
[]
%

Environment

• CLI version (output of arduino-cli version): arduino-cli Version: 0.22.0 Commit: 65f662a7 Date: 2022-05-06T11:09:35Z
• OS and platform: macOS 12.3.1 (21E258)

Additional context