Provide context necessary to resolve identity conflict.

[yyvch]

Update error messaging to be a bit more clear where the conflict of identities is coming from.

Motivation:

#8311 — when multiple packages share the same identity (for example: gh/swiftlang/swift-driver and gh/apple/swift-driver share the same identity swift-driver) then the error doesn't help to resolve the conflict. It is particularly confusing when the conflicting package is pulled from transitive dependencies. I wanted to offer more context to resolve these types of conflicts, particularly:

• how the conflict is introduced in relation to the root package
• itemize all paths for both sides of the conflict, so that users can better understand how to approach resolution, but not overwhelm the error message with too many details

Modifications:

• Rephrase error message for clarity, add a suggestion to work with maintainers of transitive dependencies.
• Reconstruct the dependency chains from roots to the conflicting packages.

Result:

Before:

'swift-build': 'swift-build' dependency on 'https://github.com/swiftlang/swift-driver.git' conflicts with dependency on 'https://github.com/apple/swift-driver.git' which has the same identity 'swift-driver'.

After:

Conflicting identity for swift-driver: dependency 'github.com/swiftlang/swift-driver' and dependency 'github.com/apple/swift-driver' both point to the same package identity 'swift-driver'. The dependencies are introduced through the following chains: (A) /users/yy/pub/swift-package-manager->github.com/swiftlang/swift-build->github.com/swiftlang/swift-driver (B) /users/yy/pub/swift-package-manager->github.com/apple/swift-driver. If there are multiple chains that lead to the same dependency, only the first chain is shown here. To see all chains use debug output option. To resolve the conflict, coordinate with the maintainer of the package that introduces the conflicting dependency.

Plus also debug logs:

debug: 'swift-build': Conflicting identity for swift-driver: chains of dependencies for https://github.com/swiftlang/swift-driver.git: [[/users/yy/pub/swift-package-manager, github.com/swiftlang/swift-build, github.com/swiftlang/swift-driver]]

debug: 'swift-build': Conflicting identity for swift-driver: chains of dependencies for https://github.com/apple/swift-driver.git: [[/users/yy/pub/swift-package-manager, github.com/apple/swift-driver]]

[yyvch]

@swift-ci please test

[yyvch]

@swift-ci please test

[yyvch]

@swift-ci please test

[bripeticca]

LGTM :)