Refactor: Extract new crate simple-git (#1304)

`binstalk-downloader` contains stuff about http(s) before the
git code is moved into it and now it becomes http and git.

While git indeed uses http stuff, which is why I decided to put
it into binstalk-downloader, it is more than just downloading
since it is stateful (can be cached locally and updated)
where as http is stateless.

Also `binstalk-downloader`'s codegen time now increases
dramatically and it also creates extra dependencies for
binstalk-fetchers, delaying its execution.

The git code also don't use anything from `binstalk-downloader`
at all, it makes sense to be an independent crate.

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

crates/binstalk-registry/src/lib.rs

@@ -19,7 +19,7 @@ use tokio::task;
 use url::{ParseError as UrlParseError, Url};
 
 #[cfg(feature = "git")]
-pub use binstalk_downloader::git::{GitError, GitUrl, GitUrlParseError};
+pub use simple_git::{GitError, GitUrl, GitUrlParseError};
 
 mod vfs;