cleanup: remove field `is_import` from `def::Export`

[jseyfried]

Remove field is_import from rustc::def::Export.
The field is only used is rustdoc and can be refactored away.

[Mark-Simulacrum]

@eddyb or @jseyfried This seems like a good candidate for an easy/mentor issue, perhaps one of you could provide what !item.is_import could be replaced with in rustdoc? The comment on the PR suggests tcx.def_key(export.def.def_id()).parent, but I'm not personally sure what one would do with that to determine whether the item is in an import statement or not.

[eddyb]

@Mark-Simulacrum You'd compare it with the DefId of the module you found the Export in.
That is, is_import signifies a re-export, where the definition is somewhere else than the module you're currently listing the Exports of.

[eddyb]

Depending on what is desired, the name of the original item might also be compared (because a reexport from the same module, aka pub use self::foo as bar;, is also possible).