Use help-fns internal symbol completion table

[wyuenho]

This PR came as a result of a package called llama that introduced a hacky macro that unfortunately results in an empty string function name that always appear at the top of the completion table.

The details of the problem is described here. Essentially, this macro abuses the fact that the empty string can be used as a symbol, which defeats every intuitive string matching and sorting algorithms, and results in an empty string function at the top of the completion table whenever helpful-callable or helpful-macro is invoked.

Recent attempts to fix this problem relied on the fact that describe-function calls all-completions down the call stack to filter this annoyance out, but helpful's helpful--read-symbol symbol uses the obarray directly without filtering. This PR addresses this issue by delegating to help--symbol-completion-table when it's available (Emacs >= 26.1).

[meedstrom]

Updating llama did not fix the issue?

EDIT: Ah, nevermind, I see you said it uses obarray directly.