Bug: Issues in External Repositories don't resolve to a valid path

[brentleyjones]

Describe the bug

The issue is the build log has paths based on the exec root, which lists external dependences at external/, instead of the Xcode visible path of $(BAZEL_EXTERNAL)/ (which is something like $(PROJECT_FILE_PATH)/rules_xcodeproj/links/external):

INFO: From Compiling Swift module @com_github_pointfreeco_swift_custom_dump//:CustomDump:
external/com_github_pointfreeco_swift_custom_dump/Sources/CustomDump/Conformances/UserNotifications.swift:154:10: warning: 'alert' was deprecated in macOS 11.0
        .alert,
         ^
external/com_github_pointfreeco_swift_custom_dump/Sources/CustomDump/Conformances/UserNotifications.swift:154:10: warning: 'alert' was deprecated in macOS 11.0
        .alert,
         ^
external/com_github_pointfreeco_swift_custom_dump/Sources/CustomDump/Conformances/UserNotifications.swift:135:32: warning: 'alert' was deprecated in macOS 11.0
          if self.rawValue == .alert {
                               ^
[51 / 60] Compiling Swift module @com_github_tuist_xcodeproj//:XcodeProj; 5s worker ... (2 actions, 1 running)

INFO: From Compiling Swift module @com_github_tuist_xcodeproj//:XcodeProj:
external/com_github_tuist_xcodeproj/Sources/XcodeProj/Workspace/XCWorkspace.swift:31:70: warning: 'self' refers to the method 'XCWorkspace.self', which may be unexpected
        let data = XCWorkspaceData(children: [.file(.init(location: .self("")))])
                                                                     ^
external/com_github_tuist_xcodeproj/Sources/XcodeProj/Workspace/XCWorkspace.swift:31:70: note: use 'XCWorkspace.self' to silence this warning
        let data = XCWorkspaceData(children: [.file(.init(location: .self("")))])
                                                                     ^
                                                                     XCWorkspace.
external/com_github_tuist_xcodeproj/Sources/XcodeProj/Workspace/XCWorkspaceDataElementLocationType.swift:65:21: warning: 'self' refers to the method 'XCWorkspaceDataElementLocationType.self', which may be unexpected
            self = .self(path)
                    ^
external/com_github_tuist_xcodeproj/Sources/XcodeProj/Workspace/XCWorkspaceDataElementLocationType.swift:65:21: note: use 'XCWorkspaceDataElementLocationType.self' to silence this warning
            self = .self(path)
                    ^
                    XCWorkspaceDataElementLocationType.

We need to post-process the output to change the paths. The same post-processing will probably be needed to fix #458.

[brentleyjones]

Depending on how we handle #400/#402, we might just stop showing these versions.