Updatable toolchain aliases

I commonly work with pinned nightlies in projects that require extra components installed, updating the pinned date in one of these projects is a lot of work since I need to find a new nightly that has all the components available. One idea I had to solve this would be to have a sort of local "channel" alias that I can manually update to the latest release of different upstream channels, and have this update use the normal check for all required compenents that happens with channels like nightly.

I think the easiest way to fully explain this would be an example of how I would expect a CLI interaction with this feature to go:

> rustup toolchain add foobar=nightly-2018-10-31
info: syncing channel updates for 'nightly-2018-10-31-x86_64-apple-darwin'
info: latest update on 2018-10-31, rust version 1.31.0-nightly (1cf82fd9c 2018-10-30)
[...]

  foobar installed - rustc 1.31.0-nightly (1cf82fd9c 2018-10-30)

> rustup component add --toolchain foobar clippy
info: downloading component 'clippy'
info: installing component 'clippy'

> rustup update foobar=nightly-2018-11-28
info: syncing channel updates for 'nightly-2018-11-28-x86_64-apple-darwin'
warning: component 'clippy' is not available anymore on target 'x86_64-apple-darwin'
error: some components unavailable for download: 'clippy'

  foobar update failed - rustc 1.31.0-nightly (1cf82fd9c 2018-10-30)

> rustup update foobar=nightly-2018-11-30
info: syncing channel updates for 'nightly-2018-11-30-x86_64-apple-darwin'
info: latest update on 2018-11-30, rust version 1.32.0-nightly (3e90a12a8 2018-11-29)
[...]

  foobar updated - rustc 1.32.0-nightly (3e90a12a8 2018-11-29)

Start by tracing the CLI flows shown for rustup toolchain add, rustup component add --toolchain, and rustup update, then inspect how channel names and pinned dates are resolved. The feature is done when a local alias can be created, manually updated to a specified upstream channel, and updates preserve the installed components by rejecting releases where required components are unavailable.