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`

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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.