doc: Reorganize the library index

Only single out std. Put everything else in a consistent list.
Ignore /build even if it’s a symlink.
auto merge of #12312 : brson/rust/doc, r=alexcrichton

Only single out std. Put everything else in a consistent list.
auto merge of #12319 : SimonSapin/rust/build-symlink, r=alexcrichton
Make priavcy checking aware that a `use` directive can point to two defintions (namespaces) with different privacy.