Move `has_eval_provider()` check to `eval_call_provider()` to make sure that
every code path calls it first.

Previously we would, when pynvim was missing, get a nice error message for
`:python3 1`, but not for `:py3file blah`.

Fixes https://github.com/neovim/neovim/issues/9485

In Vim (and some vestigial parts of Nvim) E319 was a placeholder for
ex_ni commands, i.e. commands that are only available in certain builds
of Vim.  That is obviously counter to Nvim's goals: all Nvim commands
are available on all platforms and build types (the remaining ex_ni
commands are actually just missing providers).

We need an error id for "missing provider", so it makes sense to use
E319 for that purpose.

ref #9344
ref #3577

Developer can use :checkhealth to get more details, don't need to blast
the details in the skip-message every time.

Test correctly fail for oneline ruby, python and python3.

It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.

Fixes #4822
Closes #4823

Use the existing Vimscript function provider#pythonx#Detect()
to determine whether the Neovim Python module is installed and
Python 2/3 tests can be run.

- Organize tests by logical function, not the literal impl location.
- Avoid deep nesting / hyper-hierarchy.