This commit adds support for "--watch" flag for "bundle"
and "fmt" subcommands.
In addition to this, it refactors "run --watch" command so that
module resolution will occur every time the file watcher detects
file addition/deletion, which allows the watcher to observe a file
that is newly added to the dependency as well.
This commit moves following tools into a single "tools"
module located at "cli/tools/mod.rs":
- formatter
- linter
- test runner
- coverage collector
- installer
- binary upgrader
- repl
