feature: Include source URL in lazy-lock.json, just like in nvim-pack-lock.json

[yutkat]

Did you check the docs?

• I have read all the lazy.nvim docs

Is your feature request related to a problem? Please describe.

Currently, lazy-lock.json only stores branch and commit for each plugin. This means the lockfile alone is not self-contained - you cannot determine where a plugin comes from without also referencing your plugin spec configuration.

This becomes a problem when:

• Reviewing the lockfile in a PR diff (you can't tell at a glance where a plugin comes from)
• Sharing or auditing lockfiles across machines without the accompanying spec
• Tools or scripts that want to process the lockfile independently

nvim-pack-lock.json is specified that way.
https://neovim.io/doc/user/pack/#vim.pack-lockfile

Describe the solution you'd like

Add a src field to each plugin entry in lazy-lock.json, storing the plugin's repository URL.

For example, the current format:

{
  "lazy.nvim": { "branch": "main", "commit": "abc1234" },
  "telescope.nvim": { "branch": "main", "commit": "def5678" }
}

Would become:

{
  "lazy.nvim": { "branch": "main", "commit": "abc1234", "src": "https://github.com/folke/lazy.nvim.git" },
  "telescope.nvim": { "branch": "main", "commit": "def5678", "src": "https://github.com/nvim-telescope/telescope.nvim.git" }
}

The src can be taken from the plugin's url field, or via Git.get_origin() as a fallback.

For reference, Neovim's built-in vim.pack lockfile (:h vim.pack-lockfile) already includes a src field for the same purpose, making the lockfile self-describing.

Describe alternatives you've considered

• Store only owner name: e.g. "owner": "folke". More compact but less useful — doesn't help with non-GitHub sources or when the owner alone is ambiguous.
• Store owner/repo shorthand: e.g. "source": "folke/lazy.nvim". Compact and matches the spec format users are already familiar with, but assumes GitHub.

Additional context

No response