Export the underlying SQLite library

[groue]

This pull request exports the underlying SQLite library with GRDB.

This means that importing GRDB grants access to the SQLite C interface:

// BEFORE: GRDB 4
import CSQLite   // When GRDB is included with the Swift Package Manager
import SQLCipher // When GRDB is linked to SQLCipher
import SQLite3   // When GRDB is linked to System SQLite
let sqliteVersion = String(cString: sqlite3_libversion())

// NEW: GRDB 5
import GRDB
let sqliteVersion = String(cString: sqlite3_libversion())

This completes the goal of #747, which was to make users able to change their GRDB flavor without needing to change their code.