Added "SourceryLib" product to Package.swift

[art-divin]

In order to facilitate importing of SourceryLib when Sourcery and Configuration types are needed,
"SourceryLib" product needs to be exposed.

Example:

import SourceryLib
import PathKit

public struct SourceryUseExample {
    public private(set) var text = "Hello, World!"

    public init() throws {
        let sourcery = Sourcery()
        let configuration = try Configuration(path: Path(), relativePath: Path())
        let watchers = try sourcery.processFiles(configuration.source, usingTemplates: configuration.templates, output: configuration.output, baseIndentation: configuration.baseIndentation)
        // use [FolderWatcher.Local] ...
    }
}

Here, user of SourceryLib can implement custom additonal post-processing, as well as use different techniques around calls to Sourcery as compared to using compiled executable when these types are inaccessible, as well as SouceryFramework which does not expose these types does not help in achieving such uses.