Add `cargo lucifer` subcommand

[killercup]

Add a new subcommand that brings light (get it?) to cargo's dependency resolving, answering questions like

• Why was that dependency included?
• Why was that feature on this dependency enabled?
• Which other versions were considered and why was this one chosen?

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Yes, I made this issue because I saw the last issue was number 6665. Feel free to close it as "bad joke".

[dwijnand]

I dig the joke 😄

Could this use case be satisfied by documenting something along the lines of RUST_LOG=cargo::ops::resolve=info cargo generate-lockfile?

[spacekookie]

To propose a bikeshed here, I think it should be called cargo cult

[killercup]

@dwijnand Maybe env RUST_LOG=cargo::core::resolver=trace cargo generate-lockfile if it was more pretty :D

[dwijnand]

This is how I map log levels:

• trace I'm deeeep in this module, give me lots of details
• debug I'm a cargo dev, give me the general-sense detailed log for this module
• info I'm a cargo user, looking some more info from the operations in this module

But actually, make this detail (or some of it) should be printed by cargo generate-lockfile -v.

[epage]

cargo tree I think solves

• Why was it included (cargo tree -i)
• Features (cargo tree -e features)

As for "what other versions and why this one", that gets complicated with a lot of information involved. Not really seeing a way around that than env CARGO_LOG=cargo::core::resolver=trace cargo generate-lockfile.

Probably the main thing left is documenting these resources somewhere. Like the section of the documentation on the resolver.