[xcodegen] Allow swift-plugin-server to be added

Previously we would avoid adding this since it has
an invalid module name, but we can follow the same
logic as the driver and set the module name to
`main`.

Author: hamishknight

utils/swift-xcodegen/Sources/SwiftXcodeGen/BuildArgs/SwiftTargets.swift

@@ -200,12 +200,17 @@ struct SwiftTargets {
 
     guard let buildArgs = try computeBuildArgs(for: rule) else { return }
 
-    let moduleName = buildArgs.lastValue(for: .moduleName)
+    // Pick up the module name from the arguments, or use an explicitly
+    // specified module name if we have one. The latter might be invalid so
+    // may not be part of the build args (e.g 'swift-plugin-server'), but is
+    // fine for generation.
+    let moduleName = buildArgs.lastValue(for: .moduleName) ??
+                       rule.attributes[.swiftModuleName]?.value
     guard let moduleName else {
       log.debug("! Skipping Swift target with output \(primaryOutput); no module name")
       return
     }
-    let moduleLinkName = rule.attributes[.swiftLibraryName]?.value ?? 
+    let moduleLinkName = rule.attributes[.swiftLibraryName]?.value ??
                            buildArgs.lastValue(for: .moduleLinkName)
     let name = moduleLinkName ?? moduleName
 

utils/swift-xcodegen/Sources/SwiftXcodeGen/Generator/ProjectGenerator.swift

@@ -557,8 +557,10 @@ fileprivate final class ProjectGenerator {
 
     applySubstitutions(to: &buildArgs, target: target, targetInfo: targetInfo)
 
+    // Follow the same logic as swift-driver and set the module name to 'main'
+    // if we don't have one.
     let moduleName = buildArgs.takePrintedLastValue(for: .moduleName)
-    buildSettings.common.PRODUCT_MODULE_NAME = moduleName
+    buildSettings.common.PRODUCT_MODULE_NAME = moduleName ?? "main"
 
     // Emit a module if we need to.
     // TODO: This currently just uses the build rule command args, should we