Replace dep futures-util with helpers::FuturesResolver (#765)

futures-util has too many dependencies and it contains a lot of code of
which we only use `futures_util::stream::{FuturesUnordered, StreamExt}`.

We don't even need most of the functionalities in `FuturesUnordered` as
we just need the output of first future that either returns `Err(_)` or
`Ok(Some(_))`.

So we replace it with ou own homebrew solution (~80 loc) and it's easier
to use.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

Cargo.lock

@@ -179,7 +179,6 @@ dependencies = [
  "crates_io_api",
  "detect-targets",
  "either",
- "futures-util",
  "home",
  "itertools",
  "jobslot",