Fix incorrect executable target diagnostic

The determination for executable targets isn't limited to Swift files, it will also find 'main.m' for example. This was originally reflected in the diagnostic, but #3254 updated it incorrectly.

Author: neonichu

Sources/PackageLoading/PackageBuilder.swift

@@ -953,7 +953,7 @@ public final class PackageBuilder {
             {
                 self.observabilityScope
                     .emit(
-                        warning: "'\(potentialModule.name)' was identified as an executable target given the presence of a 'main.swift' file. Starting with tools version \(ToolsVersion.v5_4) executable targets should be declared as 'executableTarget()'"
+                        warning: "'\(potentialModule.name)' was identified as an executable target given the presence of a 'main' file. Starting with tools version \(ToolsVersion.v5_4) executable targets should be declared as 'executableTarget()'"
                     )
             }
         }

Tests/PackageLoadingTests/PackageBuilderTests.swift

@@ -499,7 +499,7 @@ final class PackageBuilderTests: XCTestCase {
         )
         PackageBuilderTester(manifest, in: fs) { package, diagnostics in
             diagnostics.check(
-                diagnostic: "'exec2' was identified as an executable target given the presence of a 'main.swift' file. Starting with tools version 5.4.0 executable targets should be declared as 'executableTarget()'",
+                diagnostic: "'exec2' was identified as an executable target given the presence of a 'main' file. Starting with tools version 5.4.0 executable targets should be declared as 'executableTarget()'",
                 severity: .warning
             )
             package.checkModule("lib") { _ in }