Fix build issue

Author: Kyle-Ye

Sources/PackageLoading/Target+PkgConfig.swift

@@ -139,6 +139,8 @@ extension SystemPackageProviderDescription {
         switch self {
         case .brew(let packages):
             return "    brew install \(packages.joined(separator: " "))\n"
+        case .port(let packages):
+            return "    port install \(packages.joined(separator: " "))\n"
         case .apt(let packages):
             return "    apt-get install \(packages.joined(separator: " "))\n"
         case .yum(let packages):
@@ -156,6 +158,10 @@ extension SystemPackageProviderDescription {
             if case .darwin = platform {
                 return true
             }
+        case .port:
+            if case .darwin = platform {
+                return true
+            }
         case .apt:
             if case .linux(.debian) = platform {
                 return true
@@ -200,6 +206,8 @@ extension SystemPackageProviderDescription {
                 }
             }
             return try packages.map({ try AbsolutePath(validating: brewPrefix).appending(components: "opt", $0, "lib", "pkgconfig") })
+        case .port:
+            return []
         case .apt:
             return []
         case .yum:

Tests/PackageLoadingTests/PkgConfigTests.swift

@@ -51,6 +51,7 @@ class PkgConfigTests: XCTestCase {
                 pkgConfig: "Foo",
                 providers: [
                     .brew(["libFoo"]),
+                    .port(["libFoo"]),
                     .apt(["libFoo-dev"]),
                     .yum(["libFoo-devel"]),
                     .nuget(["Foo"]),
@@ -67,6 +68,8 @@ class PkgConfigTests: XCTestCase {
                 switch result.provider {
                 case .brew(let names)?:
                     XCTAssertEqual(names, ["libFoo"])
+                case .port(let names)?:
+                    XCTAssertEqual(names, ["libFoo"])
                 case .apt(let names)?:
                     XCTAssertEqual(names, ["libFoo-dev"])
                 case .yum(let names)?: