Refactor PackageGraph._tagReexportsFor and document

[srawlins]

Just some cleanup, possibly a performance boost:

• Remove lastExportedElement; unused.
• Do not add null values to the cycle check set.
• Track the cycle check set as a local variable, rather than a field.
• Document several related APIs.
• Rename from 'reexports' to 'exports'; I think this is just simpler.

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[srawlins]

@dart-lang/analyzer-team any volunteers to review?

[srawlins]

flutter bot is failing for known pre-existing failure: #3773

[pq]

💎