Add support for SWIFTCI_USE_LOCAL_DEPS convention (#311)

To use this package in utils/build-script pipeline

Author: kateinoigakukun

Package.swift

@@ -13,10 +13,6 @@ let package = Package(
  products:[
 .library(name:"AsyncAlgorithms", targets:["AsyncAlgorithms"]),
 ],
- dependencies:[
-.package(url:"https://github.com/apple/swift-collections.git", from:"1.1.0"),
-.package(url:"https://github.com/apple/swift-docc-plugin", from:"1.0.0"),
-],
  targets:[
 .target(
  name:"AsyncAlgorithms",
@@ -52,3 +48,14 @@ let package = Package(
 ),
 ]
 )
+
+ifContext.environment["SWIFTCI_USE_LOCAL_DEPS"]==nil{
+package.dependencies +=[
+.package(url:"https://github.com/apple/swift-collections.git", from:"1.1.0"),
+.package(url:"https://github.com/apple/swift-docc-plugin", from:"1.0.0"),
+]
+}else{
+package.dependencies +=[
+.package(path:"../swift-collections"),
+]
+}