feat: use rtp.nvim to source plugin directories

[mrcjkb]

I have extracted the rocks.api.source_runtime_dir logic into a rtp.nvim library, because it will no longer be needed by rocks.nvim once https://github.com/nvim-neorocks/rocks.nvim/pull/292 is merged.

When rocks-dev.nvim uses this library, we can deprecate the function in rocks.nvim.

[github-actions[bot]]

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