feat: add `toolchain_info` rule

BREAKING CHANGE: The `toolchain_symlink_target` no longer exports
`ToolchainInfo`/`TemplateVariableInfo`.

The `toolchain_info` can provide those providers. The `toolchain_symlink_target`
is purely for creating a symlink to a previous target. This still enables
changing the basename for multi-call binaries.

To gain the old functionality, if `basename` attribute is not being used,
`toolchain_symlink_target` can be switched directly for `toolchain_info`. If
`basename` is being used, an extra `toolchain_info` rule will need to be created
to wrap the `toolchain_symlink_target` rule.