Support -p/--package argument

[codyps]

Right now, I have to cd into a particular package within my workspace to use cargo-instruments. Other builtin cargo commands (like cargo run and cargo build) support a -p/--package argument so that one doesn't need to cd around. It would be good for cargo-instruments to support the same option.

[cmyr]

Yes, this would be nice. I actually hacked on this for a while, and it ended up not being trivial, so I didn't follow up; basically to do this correctly we would want to pretty closely match the behaviour of the built-in cargo tools, which means doing this like respecting the default-run and default-members manifest fields, and doing this requires a fairly large-scale rearchitecture. 😒