clean up requires
add more information to the doclet index (#725)

We now index doclets as follows:

+ all doclets by longname
+ all documented symbols by longname
+ all doclets with `borrowed` properties

By expanding the index, we reduce the number of times that `jsdoc/augment` and `jsdoc/borrow` must iterate over all doclets. As a result, large docsets are processed much more quickly.