Currently methods from extern crates are sometimes added to the search
index when they shouldn't be or added with the original path rather than
the reexported path. This fixes that by making sure `cache().paths` only
contains local paths like the description for it states. It also fixes a
few minor issues with link rendering and redirect generation which would
point to local crate docs even if the docs for that crate hadn't been
generated.
Also a bug with methods implemented on traits which caused wrong paths and
so dead links in the search results has been fixed.
- src links/redirects to extern fn from another crate had an extra '/'.
- src links to `pub use` of a crate module had an extra '/'.
- src links to renamed reexports from another crate used the new name
for the link but should use the original name.
