bug(yarn): workspaces are not detected if you scan non project directory

[DmitriyLewen]

Description

In cases where it is not the project directory that is being scanned, Trivy doesn't detect workspaces.

e.g.:

➜ tree
.
└── foo
    ├── bar
    │   └── generators
    │       └── package.json
    ├── package.json
    └── yarn.lock

➜ trivy -q fs -f json --list-all-pkgs .                
{
  "SchemaVersion": 2,
  "CreatedAt": "2024-02-12T13:32:24.605239+06:00",
  "ArtifactName": ".",
  "ArtifactType": "filesystem",
  "Metadata": {
    "ImageConfig": {
      "architecture": "",
      "created": "0001-01-01T00:00:00Z",
      "os": "",
      "rootfs": {
        "type": "",
        "diff_ids": null
      },
      "config": {}
    }
  }
}

➜ trivy -q fs -f json --list-all-pkgs ./foo | jq '.Results[].Packages'
[
  {
    "ID": "[email protected]",
    "Name": "hoek",
    "Identifier": {
      "PURL": "pkg:npm/[email protected]"
    },
    "Version": "6.1.3",
    "Layer": {},
    "Locations": [
      {
        "StartLine": 5,
        "EndLine": 8
      }
    ]
  }
]

Discussed in #6079