Symbols: add `fileLocal` field

Created by: chrismwendt

Previously, the symbols service was suppressing file-local definitions from ctags (e.g. private variables), which was preventing jump-to-definition from working on some tokens.

This indexes file-local definitions. Here's an example:

This also captures the boolean fileLocal from ctags and exposes it through the GraphQL API. Without this, code intel would not be able to filter out private variables defined in other files. basic-code-intel uses fileLocal and filtering was added in https://github.com/sourcegraph/sourcegraph-basic-code-intel/pull/63