Output warning instead of fail if a dependency could not be found

[Nef10]

In case a dependency is not required on all platforms, it might not
be defined - in this case show a warning instead of fail.

[github-actions]

Coverage after merging warning into main

75.09%

Coverage Report

File	Stmts	Funcs	Lines	Uncovered Lines
Sources/SwiftDependencyUpdaterLibrary
Dependency.swift	52.81%	60%	51.90%	46–64
PackageDescription.swift	71.68%	69.23%	72%	100–104, 121–123, 134–136, 162–174, 184, 99
ResolvedPackage.swift	94.20%	100%	93.55%	118, 23, 85, 95
SwiftDependencyUpdater.swift	100%	100%	100%
SwiftPackage.swift	94.94%	83.33%	95.89%	69, 75
SwiftPackageUpdate.swift	79.45%	100%	77.27%	41, 50, 54, 58–69
Update.swift	58.23%	75%	57.33%	41, 43–73
Sources/SwiftDependencyUpdaterLibrary/Extensions
Collection.swift	100%	100%	100%
String.swift	100%	100%	100%
Version.swift	0%	0%	0%	11–17, 19–32, 36–43
Sources/SwiftDependencyUpdaterLibrary/Services
Git.swift	88%	83.33%	88.89%	62–64, 71–74
GitHub.swift	71.88%	80%	71.19%	61, 64, 68–79, 84–86
Sources/SwiftDependencyUpdaterLibrary/SubCommands
GitHubCommand.swift	82.61%	37.50%	86.90%	48–53
ListCommand.swift	78.38%	20%	87.50%
UpdateCommand.swift	65.96%	20%	71.43%	22–27, 33–34