Add a way to check whether a package is published to the index

[smoelius]

Describe the bug
I have multiple interdependent packages that I keep in version sync and publish together. If I try to publish a dependent package after a package it depends on too quickly, the process can sometimes fail.

More specifically, I have a set of packages that I publish with a script. They are published in an order that allows all of their dependencies be met, e.g., if A depends upon B, then B is published before A.

The packages' requirements on one another are strict: If A depends upon B, and B is at version X, then A's requirement for B is =X.

Finally, the packages are kept in version sync: they'll all be at some version X, I'll change all of their versions to Y, change all of their respective requirements to =Y, and then run the script.

The problem is: cargo publish will sometime fail because A's requirement for B cannot be met, even though B was published just moments ago.

I tried two workarounds for this:

1. Check that curl "https://crates.io/api/v1/crates/B" | jq -r '.crate | .newest_version' is Y.
2. Check that curl "https://crates.io/api/v1/crates/B/versions" | jq -r '.versions | map(.num) | .[]' includes Y.

Neither seems to work. For example, B's .newest_version can be Y, yet A's requirement for version Y of B can fail to be met.

My current workaround is to just re-run the script, but I would prefer a more elegant solution.

So, my question is: is there a reliable way to tell when version Y of package B has been published? That is, is there a check that, when successful, ensures a requirement for version Y of package B will be met?

To Reproduce
Steps to reproduce the behavior:

• Create two interdependent packages A and B with the same version Y, and such that A's requirement for B is =Y.
• Publish version Y of B.
• Immediately try to publish version Y of A.

Expected behavior
In the above scenario, I would expect cargo publish to succeed for for version Y of package A once any of the following become true:

• Version Y of B is published.
• curl "https://crates.io/api/v1/crates/B" | jq -r '.crate | .newest_version' is Y.
• curl "https://crates.io/api/v1/crates/B/versions" | jq -r '.versions | map(.num) | .[]' includes Y.

Screenshots
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Desktop (please complete the following information):
The scripts are run from a GitHub action in ubuntu-latest.

Smartphone (please complete the following information):
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Additional context
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