Open docs with opener crate

[Seeker14491]

The idea is to pull the doc-opening functionality from Cargo and rustup out into its own crate, as discussed in rust-lang/cargo#5701 (comment).

As I mentioned in Cargo's pull request, the handling of the $BROWSER environment variable changes on Linux.

[pickfire]

@Seeker14491 Why not use open crate instead, it is quite minimal and does not depend on failure. (did 1.0 major release)

[Seeker14491]

An advantage of opener is better error messages on Mac and Linux. Additionally, the main motivation of making the opener crate was to unify cargo and rustup's documentation-opening code. Using two separate crates would defeat the purpose. It's been integrated into cargo, and seems to be working well.

However, a cargo issue has been opened where it seems we've decided we want to explicitly check the $BROWSER environment variable for the purpose of overriding the browser used to open the docs. This should probably be implemented before this pull request is merged.

[pickfire]

@Seeker14491 Ah, I see that cargo already uses opener. There seems to be an issue to use $BROWSER in open since 2017 though. Byron/open-rs#8

I think both cargo and rustup uses way different crate, both are different projects, I don't see how it matters much to use different crate since the usage of rustup and cargo is very different.

Say, rustup uses rustup component add --toolchain nightly but cargo have cargo +nighty. As well, the frequency of usage between rustup differs quite a lot from cargo (it even have a fancy progress bar).

[nrc]

Closing for now due to lack of review bandwidth. I would like to share more code between Cargo and Rustup though.