fix(loader): look for themes on package.path

[mrcjkb]

Fixes nvim-neorocks/rocks.nvim#536.

When lualine.nvim is installed as a luarocks package¹, the loader can't find the themes on the runtimepath, because
Neovim doesn't keep the runtimepath and package.path in sync.
This PR introduces a fallback search of the package.path if a theme can't be found on the runtimepath.

Footnotes

1. luarocks installs packages with a non-flat hierarchy, with lua and lib directories in a separate tree. ↩

[shadmansaleh]

Thanks. neorocks looks interesting.