Take advantage of weak features in Rust 1.60 for TLS enablement

[MarijnS95]

Previously it was impossible to have a unified feature named rustls or native-tls that would turn on the respective TLS backend in the chosen transport (reqwest or ureq) as a feature of <crate>/tls would implicitly turn on <crate>. Since Rust 1.60 it is now possible to specify this crate-feature enablement through the use of the question mark in <crate>?/tls, which will only enable the tls feature if <crate> was enabled through other means (another feature).

Secondly we can now also let optional crates have the same name as a feature, and use dep:<crate> to refer to the crate instead of the dependency, making for a more pleasant experience without renames to underscore suffixes.

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Filing this as draft just to get the word out and discuss this for a bit, as it will only be mergeable in 6 months in accordance with the MSRV policy. It goes without saying that this is a breaking change.

Also note that I haven't renamed other optional crates like anyhow_, log_ etc yet, until this is accepted as a (future) solution.

[flub]

Agree that this would be a nice improvement. Though you correctly note that we can't just change this right now and need to wait a while.

[Swatinem]

@MarijnS95 Its been a while ;-)
We have now bumped MSRV to 1.60 in master, so the PR is unblocked now.
Are you up to update it? Otherwise I might take over the PR in the coming days.

[MarijnS95]

@Swatinem cool, sure! That was a bit more work than expected, with both conflicts and more crates in other Cargo.toml that were imported with _ suffixes (and I either missed them when initially creating this PR, or they were added recently).

Let's see how the CI likes this :)

[Swatinem]

nice!

there is a few places that still has pkg = { package = "pkg", …

It might be worth removing that, but its also fine to merge like this.

[MarijnS95]

@Swatinem Good catch, looks like I missed those while resolving conflicts!

There was also a stray anyhow_ dev-dependencies import remaining, but it's not optional so shouldn't conflict with the anyhow feature.

[codecov-commenter]

Codecov Report

Merging #454 (cd2ca25) into master (89b31f8) will increase coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head cd2ca25 differs from pull request most recent head 7b1f3cd. Consider uploading reports for the commit 7b1f3cd to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
+ Coverage   78.60%   78.65%   +0.05%     
==========================================
  Files          76       76              
  Lines        9206     9192      -14     
==========================================
- Hits         7236     7230       -6     
+ Misses       1970     1962       -8