[Blueprints] Accept branch names and tags in git:directory resource

[adamziel]

Motivation for the change, related issues

Enables sourcing data from arbitrary branches and tags via Blueprints.

Before this PR, you had to use a ref name that's either HEAD or a commit hash. With this PR, you can also reference branches by their names and tags. Also, the path property is no longer required and defaults to /.

Testing Instructions (or ideally a Blueprint)

Run this Blueprint and confirm it installs and activates the Blocky formats plugin from its Git repo:

{
  "login": true,
  "landingPage": "/wp-admin/post-new.php",
  "steps": [
    {
      "step": "installPlugin",
      "options": {
        "activate": true,
        "targetFolderName": "blocky-formats"
      },
      "pluginData": {
        "resource": "git:directory",
        "url": "https://github.com/dmsnell/blocky-formats.git",
        "ref": "trunk"
      }
    }
  ],
  "preferredVersions": {
    "php": "8.3",
    "wp": "latest"
  },
  "features": {}
}