refactor: rework toolchain registration for WORKSPACE and bzlmod users

[kormide]

Stop automatically registering all of the toolchains in bazel-lib for WORKSPACE users. Make it easier to override toolchain versions under bzlmod.

Closes #593.

Type of change

• Refactor (a code change that neither fixes a bug or adds a new feature)

For changes visible to end-users

• Breaking change (this change will force users to change their own code or config)
• Relevant documentation has been updated
• Suggested release notes are provided below:

• BREAKING: Toolchains are no longer automatically registered for WORKSPACE users. Users must call the aspect_bazel_lib_register_toolchains macro or call the register_xyz_toolchain macros individually. See Release for snippets.
• BREAKING: The host repository has been moved from the extension ext into a differed module extension named host.

Test plan

• Covered by existing test cases
• New test cases added
• Manual testing; please provide instructions so we can reproduce:

[alexeagle]

I don't think we need to completely stop registering toolchains. Our goal is just to make sure that users don't download unnecessary binaries, and are able to override our toolchain with whatever one they choose, right?

https://github.com/bazelbuild/rules_go/blob/master/docs/go/core/bzlmod.md#registering-go-sdks suggests we should be able to see how they avoid registration taking precedence over users.

[kormide]

Updated. Under bzlmod, continue autoregistering toolchains as this does not interfere with the ability for someone to override a toolchain in the root module (see https://bazelbuild.slack.com/archives/C014RARENH0/p1696893626545609). Toolchains can be selectively overridden at different versions.

For WORKSPACE users, added an aspect_bazel_lib_register_toolchains macro which can be used to perform all of the registration. Users can optionally register them selectively with the individual toolchain macros.

Made extension_utils private while we work out its utility.