plugins: Pass correct dependency origin information to plugins (#7506)

### Motivation:

SwiftPM sends information about the origin of each package dependency to
the plugin process, but the `PluginContext` passed to the plugin
function incorrectly shows all origins as `root`.

### Modifications:

The necessary information is included correctly in the serialized
structure sent to the plugin by SwiftPM. The startup wrapper code in the
plugin process correctly deserializes it but does not copy it into the
final `PluginContext` struct which is passed to the plugin function;
instead, `origin` is hard-coded to `.root`.

This change copies the dependency information into the final context
struct.

### Result:

Plugins receive correct information about package dependencies -
`local`, with a local path, or `repository` with a URL, version number
and revision hash.

Author: euanh

Sources/PackagePlugin/PluginContextDeserializer.swift

@@ -251,12 +251,22 @@ internal struct PluginContextDeserializer {
         }
         let products = try wirePackage.productIds.map { try self.product(for: $0) }
         let targets = try wirePackage.targetIds.map { try self.target(for: $0) }
+        let origin: PackageOrigin = switch wirePackage.origin {
+            case .root:
+                .root
+            case .local(let pathId):
+                try .local(path: url(for: pathId).path)
+            case .repository(let url, let displayVersion, let scmRevision):
+                .repository(url: url, displayVersion: displayVersion, scmRevision: scmRevision)
+            case .registry(let identity, let displayVersion):
+                .registry(identity: identity, displayVersion: displayVersion)
+        }
         let package = Package(
             id: wirePackage.identity,
             displayName: wirePackage.displayName,
             directory: Path(url: directory),
             directoryURL: directory,
-            origin: .root,
+            origin:  origin,
             toolsVersion: toolsVersion,
             dependencies: dependencies,
             products: products,

Tests/CommandsTests/PackageCommandTests.swift

@@ -1890,6 +1890,11 @@ final class PackageCommandTests: CommandsTestCase {
                                 print("  \\(file.path): \\(file.type)")
                             }
                         }
+
+                        // Print out the dependencies so that we can check them.
+                        for dependency in context.package.dependencies {
+                            print("  dependency \\(dependency.package.displayName): \\(dependency.package.origin)")
+                        }
                     }
                 }
                 """
@@ -1986,6 +1991,12 @@ final class PackageCommandTests: CommandsTestCase {
                 let (stdout, _) = try SwiftPM.Package.execute(["mycmd"], packagePath: packageDir)
                 XCTAssertMatch(stdout, .contains("Initial working directory: \(workingDirectory)"))
             }
+
+            // Check that information about the dependencies was properly sent to the plugin.
+            do {
+                let (stdout, _) = try SwiftPM.Package.execute(["mycmd", "--target", "MyLibrary"], packagePath: packageDir)
+                XCTAssertMatch(stdout, .contains("dependency HelperPackage: local"))
+            }
         }
     }