For a large module, opening the index was populating tables with
entries for every package in the module. If we are only using a small
number of those packages, this is wasted work that can dwarf the
benefit from the index.
This CL changes the index reader to avoid loading all packages
at module index open time. It also refactors the code somewhat
for clarity.
It also removes some duplication by defining that a per-package
index is a per-module index containing a single package, rather
than having two different formats and two different decoders.
It also changes the string table to use uvarint-prefixed data
instead of having to scan for a NUL byte. This makes random access
to long strings more efficient - O(1) instead of O(n) - and can significantly
speed up the strings.Compare operation in the binary search looking
for a given package.