Auto merge of #11032 - lqd:priority_pending_queue, r=Eh2406

Auto merge of #11032 - lqd:priority_pending_queue, r=Eh2406

Take priority into account within the pending queue

This is the PR for the work discussed in [this zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/pending.20queue.20scheduling.20experiments) and whose detailed description and some results are available [here](https://github.com/lqd/rustc-benchmarking-data/tree/main/experiments/cargo-schedules/pending-queue-sorted) with graphs, summaries and raw data -- much of which was shown in the thread as well.

Units of works have a computed priority that is used in the dependency queue, so that higher priorities are dequeued sooner, as documented [here](https://github.com/rust-lang/cargo/blob/996a6363ce4b9109d4ca757407dd6dcb4805c86f/src/cargo/util/dependency_queue.rs#L34-L35).

This PR further applies that principle to the next step before being executed: if multiple pieces of work are waiting in the pending queue, we can sort that according to their priorities. Here as well, higher priorities should be scheduled sooner.

They are more often than not wider than pure chains of dependencies, and this should create more parallelism opportunities earlier in the pipeline: a high priority piece of work represents more future pieces of work down the line, and try to sustain CPU utilization longer (at the potential cost of this parallelism being distributed differently than today, between cargo invoking rustc and rustc's own codegen threads -- when applicable).

This is a scheduling tradeoff that behaves differently for each project, machine configuration, amount of available parallelism at a given point in time, etc, but seems to help more often than hinders: at low-core counts and with enough units of work to be done, so that there is jobserver token contention where choosing a "better" piece of work to work on next may be possible.

There's of course a bit of noise in the results linked above and 800 or so of the most popular crates.io crates is still a limited sample, but they're mostly meant to show a hopefully positive trend: while there are improvements and regressions, that trend looks to be more positive than negative, with the wins being more numerous and with higher amplitudes than the corresponding losses.

(A report on another scheduling experiment -- a superset of this PR, where I also simulate users manually giving a higher priority to `syn`, `quote`, `serde_derive` -- [is available here](https://github.com/lqd/rustc-benchmarking-data/tree/main/experiments/cargo-schedules/pending-queue-prioritized) and also improves this PR's results: the regressions are decreased in number and amplitude, whereas the improvements are bigger and more numerous. So that could be further work to iterate upon this one)

Since this mostly applies to clean builds, for low core counts, and with a sufficient number of dependencies to have some items in the pending queue, I feel this also applies well to CI use-cases (esp. on the free tiers).

Somewhat reassuring as well, and discussed in the thread but not in the report: I've also tried to make sure cargo and bootstrapping rustc are not negatively affected. cargo saw some improvements, and bootstrap stayed within today's variance of +/- 2 to 3%. Similarly, since 3y old versions of some tokio crates (`0.2.0-alpha.1`) were the most negatively affected, I've also checked that recent tokio versions (`1.19`) are not disproportionately impacted: their simple readme example, the more idiomatic `mini-redis` sample, and some of my friends' tokio projects were either unaffected or saw some interesting improvements.

And here's a `cargo check -j2` graph to liven up this wall of text:

![some results of `cargo check -j2`](https://github.com/lqd/rustc-benchmarking-data/raw/main/experiments/cargo-schedules/pending-queue-sorted/images/check-j2-sorted.png)

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I'm not a cargo expert so I'm not sure whether it would be preferable to integrate priorities deeper than just the dependency queue, and e.g. have `Unit`s contain a dedicated field or similar. So in the meantime I've done the simplest thing: just sort the pending queue and ask the units' priorities to the dep queue.

We could just as well have the priority recorded as part of the pending queue tuples themselves, or have that be some kind of priority queue/max heap instead of a Vec.

Let me know which you prefer, but it's in essence a very simple change as-is.