[BUG] cant install asynq/tools/asynq@v0.21.0

[Hans941]

Describe the bug
I want install tools/asynq. But my go version is not 1.18 or higher.So I want install an old version.

To Reproduce

➜  ~ go version                                              
go version go1.17.11 darwin/arm64
➜  ~ go install github.com/hibiken/asynq/tools/asynq@v0.21.0
go install: github.com/hibiken/asynq/tools/asynq@v0.21.0: module github.com/hibiken/asynq@v0.21.0 found, but does not contain package github.com/hibiken/asynq/tools/asynq
``

**Expected behavior**
Install an old version of asynq/tools/asynq

**Environment (please complete the following information):**
 - OS: macOs

[hibiken]

This may be due to the fact that we are currently releasing the CLI together with the library itself. We may need to consider decoupling the two in the future.

[kamikazechaser]

@hibiken Can we extract x and tools into their own repos and tag them with new versions?

This is a breaking change (We have a warning on the README in any case) but improves the support around them.

Maybe a new repo: hibiken/asynq-utils, hibiken/asynq-extra and put every thing under a pkg folder?