add symbol index component

[x] building index from scratch
[x] saving/loading index cache to disk
[x] implement with find all symbols
[x] suggest auto-completion with auto-importing un-imported modules - fix #83
- [x] respect renamed imports
- [ ] suggest & add local import if there is an import in another (unreachable) scope
[x] implement for auto-import suggestion on diagnostic "undefined symbol xyz"
[x] add basic find-references support
[x] make this index building and management optional (add code-d setting)
[x] recurse through public import / package import
[x] mark modules with mixin as potentially always included (recursively)

Pure-D / serve-d