Remove unavailable toolchains from `target list`

[Geemili]

This should close issue #299

[brson]

This is almost there, but not quite. The TargettedPackage represented by targ_pkg is the "rust" package, but you need the TargettedPackage for each of the components of "rust" (the "rust" package is made up of other components like "rust-std", each with their own packages).

So in both these loops over components you're going to need to take the pkg and target fields of the component and then look up the targetted package in the manifest for that component. This is done the same way as the lookups for the previous rust_pkg and targ_pkg variables.

Thanks!

[Geemili]

Thanks for the advice! I think I fixed it now, but I'm not sure.

Also, as a side note, how do I run rustup instead of the installer? I haven't been able to find that information anywhere.

[brson]

Thanks @Geemili!