feat: support nested go modules (monorepo)

[troyanov]

In case of a monorepo project (e.g. Python + Go) go.mod might be located somewhere in nested directories, but not in the project root.

Usage of Path:new('.') won't work well in that case, because it is actually a current working directory (cwd).

Change to Path:new(vim.fn.expand("%:p")) adds nested modules support, since go.mod search will be started from the location of the current file (opened in the buffer)

https://neovim.io/doc/user/builtin.html

% current file name :p expand to full path