Refactor, improvements and bugfix for resolution and installation process (#619)

* Refactor: Extract new mod `ops::resolve::resolution`
* Refactor: Extract new type `ResolutionFetch`
* Refactor: Extract new type `ResolutionSource`
* Improve `Resolution::print`: Provides more details on which crate
   is the resolution for.
* Make `Resolution::print` a pub fn
* Refactor: Extract new fn `ResolutionFetch::install`
* Refactor: Extract new async fn `ResolutionSource::install`
* Optimize `ResolutionSource::install` avoiding `OsStr::to_string_lossy`.
* Add new dep command-group v2.0.0 to binstalk with feat with-tokio
* Fix `ResolutionSource::install`: Use `AsyncCommandGroup::group_spawn`
   instead of `tokio::process::Command::spawn` to ensure all the processes
   spanwed by the `cargo` process can be terminated with one signal sent to
   the `cargo` process.
* Fix printing resolution: Make sure they are printed without interleaving
* Refactor `entry::install_crates`
* Improve dry-run output for `ResolutionSource::install`
* Refactor: Extract new fn `ResolutionFetch::print`
* Refactor: Extract new fn `ResolutionSource::print`
* Optimize `Resolution`: Box unit variant `Fetch`
* Improve formatting of `tokio::process::Command`
   Prints out sth like `cargo install ...` instead of the `Debug::fmt`.
* Improve dry-run output

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

Cargo.lock

@@ -168,6 +168,7 @@ dependencies = [
  "binstalk-downloader",
  "binstalk-types",
  "cargo_toml",
+ "command-group",
  "compact_str",
  "crates_io_api",
  "detect-targets",
@@ -474,6 +475,18 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "command-group"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e98782b6b757b633bd1da9d1a4ee7d323a6bd205a32b1c242d5e56e7092dfd3f"
+dependencies = [
+ "async-trait",
+ "nix",
+ "tokio",
+ "winapi",
+]
+
 [[package]]
 name = "compact_str"
 version = "0.6.1"
@@ -1451,6 +1464,18 @@ dependencies = [
  "tempfile",
 ]
 
+[[package]]
+name = "nix"
+version = "0.26.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "46a58d1d356c6597d08cde02c2f09d785b09e28711837b1ed667dc652c08a694"
+dependencies = [
+ "bitflags",
+ "cfg-if",
+ "libc",
+ "static_assertions",
+]
+
 [[package]]
 name = "normalize-path"
 version = "0.2.0"
@@ -2169,6 +2194,12 @@ version = "0.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6e63cff320ae2c57904679ba7cb63280a3dc4613885beafb148ee7bf9aa9042d"
 
+[[package]]
+name = "static_assertions"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
+
 [[package]]
 name = "strsim"
 version = "0.10.0"