pkg/codesearch: add more features

[dvyukov]

We need to add more features/functions that can be used by agents. There are some that look mandatory to provide at least complete capabilities related to code navigation:

• list directory contents (what source files exist)
• provide info about all other entity kinds (structs, typedefs, macros, enums, fields, etc)
• cross-references (where else a particular entity is used)

There are many more auxiliary features. Here are some ideas, but we need to evaluate them better b/c implementing every possible idea won't work:

• call graph info, call chains between 2 points, resolution of targets of function pointers
• allocation/free sites for the given type
• some info about locking (what locks taken in what functions, what they protect, etc)