Support test target for local Swift Package (#1169)

* support local Swift Package test case into test scheme

* update test

* add test

* update CHABGELOG.md

* Update CHANGELOG.md

* revert resolved package test

* Update Sources/XcodeGenKit/SchemeGenerator.swift

Co-authored-by: Kohki Miki <giginet.net@gmail.com>

* make TargetReference convert from new JSON format

* add .package for location of target reference

* receive target reference format at target of scheme

* update test

* update XcodeProj

* add test and fix small bugs

* update docs

* support multiple style of coverageTargets

* add edge case of parsing test targets

* fix docs

* Update Docs/ProjectSpec.md

Co-authored-by: Yonas Kolb <yonaskolb@users.noreply.github.com>

* create TestableTargetReference for not making API complex

* fix code format

* fix parameter name to Testable Target Reference

* support directly writing key of Testable Target Reference

* fix compile error in build

Co-authored-by: Kohki Miki <giginet.net@gmail.com>
Co-authored-by: Yonas Kolb <yonaskolb@users.noreply.github.com>