Auto merge of #5398 - dwijnand:drop-legacy-paths, r=matklad

Drop legacy path support under Rust edition 2018 (or later)

builds on #5335

submitted for early feedback: wdyt @matklad? is this what you had in mind? what should change? what should be added? how should we test this? is the current (2015) messaging enough to drop it in 2018?

r? @matklad

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