Improve component installation ordering

[FranciscoTGouveia]

Currently, when installing a toolchain, components are processed in alphabetical order, which may not be the most efficient approach (related Zulip discussion).

In practice, users often end up waiting for the rustc component download near the end of the installation process before the toolchain can complete its full installation.
Downloading and installing rustc first could allow the remaining components to be processed concurrently afterward, reducing the overall toolchain installation time.

[rami3l]

Also, given the current transaction system, if rustc-docs is installed before rust-docs, conflicts might happen during the transaction of the latter :o