Various improvements to team member validation

[steffahn]

• refactor the duplication check
• add checks for duplicate roles of a person in a team
• check against invalid roles of alumni

supersedes both #2701 and #2702

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github

[steffahn]

By running the overlap check only on the already individually-deduplicated list of members+alumni, something like e.g. a duplicate alumni won't result in a sequence of multiple of error messages as it currently does

before

[ERROR rust_team::validate] validation error: team `myteam` has duplicate alumni: foobar
[ERROR rust_team::validate] validation error: team `myteam` has duplicate leads + alumni: foobar
[ERROR rust_team::validate] validation error: team `myteam` has duplicate members + alumni: foobar

after

[ERROR rust_team::validate] validation error: team `myteam` has duplicate alumni: `foobar`

---

The choice of IndexMap and BTreeSet in the new no_duplicates code produces less arbitrary orderings, no longer based on hashes but on order of (first) occurrence in the files.

[steffahn]

Prepending a revert commit of #2701 and rebasing to remove the merge conflict.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[steffahn]

All code that #2701 (or its revert) touches are changed here anyway, so I might as well squash it then.