Consider ways to use local Swift SDK package in toolkit

[yo1995]

arcgis-maps-sdk-swift-toolkit/Package.swift

Lines 30 to 32 in fb8b3e4

	dependencies: [
	.package(url: "https://github.com/Esri/arcgis-maps-sdk-swift", .upToNextMinor(from: "200.0.0-beta"))
	],

Currently the toolkit points to the latest release version of the Swift SDK. While we are not allowed to put the internal package URL in a public repo, we can still allow internal users to build the toolkit using the local source code package.

Apple has many packages that use a environment variable SWIFTCI_USE_LOCAL_DEPS (ex1, ex2) to switch between public package and local folder. We may consider sth similar after hearing feedback from internal users.