Magic support for cargo xtask

[passcod]

One annoyance with cargo xtask patterns is that in many cases, the xtask command only works from the root of the workspace. Cargo Watch could solve this, though, because things running inside Cargo Watch are already wrapped by nature of running processes from other processes, so we can do magic as part of that wrapping.

For example, we could parse cargo config and figure out if an xtask alias is defined, and if it is, we could replace it in the command environment (with CARGO_ALIAS_XTASK=...) with something that always works from the particular working directory we're invoking the command from.

[This issue is not a confirmed feature, and may not be implemented.]