Report module reexports in rustdoc

[brson]

rustdoc lost this ability, presumably with the resolve rewrite. I want to do it differently next time: for each module add another section that lists reexports and links to their original definition.

[brson]

Discovering reexports requires running resolve to completion, and that requires loading the already compiled binaries of all dependencies, which is not the place of a documentation tool.

Perhaps they can be annotated manually.

[graydon]

If a crate saved its reexports in metadata tables, would this be sufficient?

[graydon]

(nominating for feature-complete)

[brson]

@graydon It would not be necessary. Rustdoc can't figure out reexports because it can't depend on having access to compiled crates. To read the metadata out of the crates, the crates would need to be precompiled before running rustdoc, but if that were the case then rustdoc wouldn't have this problem anyway because it could just run resolve.

[graydon]

dupe of / continued in #5486