[xtask] Add support for xtask doc commands

[jamesmunns]

This PR factors out a passthrough helper, and uses it for doc and clippy invocations.

Closes #2422

This somewhat punts on the flexibility items that @hawkw raised in #2422, as supporting both "before the --" and "after the --" args, in addition to the variable-length task list, was trickier than I wanted to get involved with in clap. That being said: it's not a feature anyone has needed yet, so I feel comfortable leaving that part as a todo.

[jamesmunns]

Ah, I did just notice one limitation of my approach:

Like xtask clippy, it is bound to task invocations, not package limitations. This may not actually fulfull the original request by @hawkw.

For example, this does work:

cargo xtask doc app/cosmo/rev-b.toml packrat -- --open

But this does not work:

cargo xtask doc app/cosmo/rev-b.toml ereports -- --open

edit: This does work, but requires some knowledge of which crate is used in which app+task:

cargo xtask doc app/grapefruit/rev-b.toml ereportulator -- -p ereports --open

[mkeeter]

(I haven't read the implementation yet, but here's some thoughts on CLI stuff)

On one hand, it only makes sense to document packages in the context of a specific (manifest, task) tuple – tasks may enable / disable features in downstream crates which change docs.

On the other hand, it would be nice if this Just Worked™ in a sensible way. We do something similar in cargo xtask lsp by hard-coding a (increasingly out-of-date) set of manifests to check, then finding the first task in one of those manifests which uses the crate containing a source file.

I can think of a few ways to organize this at the CLI level:

• cargo xtask doc app/cosmo/rev-b.toml [--task task] [target], where target is treated as either a crate or a task depending on whether --task is present (and if --task is present but target is absent, we target the task)
• cargo xtask doc app/cosmo/rev-b.toml [--task task] [--crate crate], where we either document the task or the crate. This could also use the "find the first task which enables that package" strategy if --task is missing

I lean slightly towards the former, because it matches our existing CLI (e.g. cargo xtask build and the current cargo xtask clippy), but could be persuaded otherwise.