This patch adds support for completion candidates for functions/macros
imported via `import`, modules aliases via `alias`, module attributes,
and any of the above when injected via a macro such as `use`.

However, this is powered by new APIs and compiler changs that will be
available in Elixir 1.17, so when completions are enabled, we will use a
bundled 1.17 runtime of Elixir, instead of the ELixir in the user's
path.

This is a tradeoff, but I think one that is worthwhile in the name of
progress and improving the language and ecosystem.

Once completions exit experimental status, this means that Next LS will
always run with a bundled copy of Elixir of Elixir unless the user's
local copy is sufficiently new. This can be controlled via a setting.