The benefit of this is that users only pay for what they use. If e.g.
only `vim.lsp.buf_get_clients()` is called then they don't need to load
all modules under `vim.lsp` which could lead to significant startuptime
saving.

Also `vim.lsp.module` is a bit nicer to user compared to
`require("vim.lsp.module")`.

This isn't used for some nested modules such as `filetype` as it breaks
tests with error messages such as "attempt to index field 'detect'".
It's not entirely certain the reason for this, but it is likely it is
due to filetype being precompiled which would imply deferred loading
isn't needed for performance reasons.

Fixes a regression from 5e5f5174e3faa862a9bc353aa7da41487911140b

Until that commit we had a logic like this:
`local prefix = startbyte and line:sub(startbyte + 1) or line_to_cursor:sub(word_boundary)`

The commit changed the logic and no longer cut off the line at the cursor,  resulting in a prefix that included trailing characters

Fixes https://github.com/neovim/neovim/issues/25177

I initially wanted to split this into a refactor commit to make it more
testable, but it appears that already accidentally fixed the issue by
normalizing lnum/col to 0-indexing

To reduce cross-chatter between modules and for https://github.com/neovim/neovim/issues/25272
Also preparing for https://github.com/neovim/neovim/issues/25714