:silent does not silence this message, even :redir does not consume it.
But execute() _does_ consume it, which interferes with the current
implementation of health.vim. It's prudent to avoid it in any case, even
if the implementation of health.vim changes in the future.

We can add this later if it is proven necessary, but it should not be
because:

1. User can run a subset of checkers via `:CheckHealth plugin1, ...,`
2. Healthcheck is a very rare operation. Optimizing it is not worth the
   code/API complexity.

To healthcheck the "foo" plugin:
    :CheckHealth foo
To healthcheck the "foo" and "bar" plugins:
    :CheckHealth foo bar
To run all auto-discovered healthchecks:
    :CheckHealth

- Overlay markdown syntax/filetype, don't invent new filetypes/syntaxes.
- migrate s:check_ruby()
- s:indent_after_line1
- Less-verbose output

- Use execute() instead of redir
- Fixed logic on suboptimal pyenv/virtualenv checks.
- Move system calls from strings to lists. Fixes #5218
- Add highlighting
- Automatically discover health checkers
- Add tests

Helped-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Helped-by: Tommy Allen <tommy@esdf.io>

Closes #4932

This makes stderr and exit callbacks work for rpc jobs

If the base directory does not exist, let mkdir(...,'p') create it.

Without the for-loop it is easier to follow, more explicit, and fewer
lines.

Uses $NVIM_RPLUGIN_MANIFEST if available