Vim added more flags for testing yet more dimensions of its Python
situation.  Handle those in eval_has_provider().

vim-patch:8.0.1436: not enough information about what Python version may work
Problem:    Not enough information about what Python version may work.
Solution:   Add "python_compiled", "python3_compiled", "python_dynamic" and
            "python3_dynamic" values for has().

ref: https://github.com/neovim/neovim/pull/10942#issuecomment-529479500

Eliminate `expect_err` in favor of `pcall_err` + `eq` or `matches`.

This is better practice in general, and allows to remove the "helptags
ALL" hacks.

Ref: https://github.com/neovim/neovim/issues/8824
Ref: https://github.com/neovim/neovim/commit/f1b67c3453c

* Makefile: fix dependencies with regard to helptags

- use the file as the main target to avoid unnecessary triggering
- use "make oldtest" on Travis to ensure it gets built

Do not incorrectly prefer python2 if python3 is working.
fixes #10758

We don't want to retry autoload sourcing (slow) for every random has()
query that finds it way to eval_call_provider().

Instead of deciding provider status in eval_has_provider, move the
decision to the provider Vim scripts.

Previously, provider loading worked as follows:

1. eval_has_provider() verified provider availability by searching for
   the provider#providername#Call function and cached this verificaion as a static
   variable for some providers
2. providers short-circuited on loading to prevent the definition of the
   Call function (with the exception of the node provider that did not)

This commit changes the expected interface between nvim and its
providers to facilitate provider reloading, by splitting the
verification of the provider from the availability of the Call function.

eval_has_provider() now checks for a provider#providername#enabled
variable. It is up to the provider script to set this to 0 or 1
accordingly. eval_call_provider() remains unchanged.

All providers hosting a Call function were updated to respect this.

The clipboard provider now has a Reload function to reload the
provider.

Avoid hyper-granularity. Don't need subdirectories for every little
thing.