[main] Enable -user-module-version for 6.0 (#7263)

Summary:
`#if canImport(LibFoo, _version: "1.2.3")` is not currently supported by
SwiftPM, as this feature was gated on `.vNext`, even though it landed
before SwiftPM 5.10 branched off.

Steps To Reproduce:
1. Build a package with `// swift-tools-version: 6.0` and try to use
`#if canImport(LibFoo, _version: "1.2.3")` for a dependency LibFoo.

Results:
The availability check won't work because `-user-module-version` option
is not passed correctly to the build system.

Author: MaxDesiatov

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -621,7 +621,12 @@ public final class SwiftTargetBuildDescription {
         }
 
         // Pass `-user-module-version` for versioned packages that aren't pre-releases.
-        if let version = package.manifest.version, version.prereleaseIdentifiers.isEmpty, version.buildMetadataIdentifiers.isEmpty, toolsVersion >= .vNext {
+        if
+          let version = package.manifest.version, 
+          version.prereleaseIdentifiers.isEmpty &&
+          version.buildMetadataIdentifiers.isEmpty &&
+          toolsVersion >= .v6_0
+        {
             args += ["-user-module-version", version.description]
         }
 

Tests/BuildTests/BuildPlanTests.swift

@@ -6064,7 +6064,7 @@ final class BuildPlanTests: XCTestCase {
                     displayName: "ExtPkg",
                     path: "/ExtPkg",
                     version: "1.0.0",
-                    toolsVersion: .vNext,
+                    toolsVersion: .v6_0,
                     products: [
                         ProductDescription(name: "ExtPkg", type: .library(.automatic), targets: ["ExtLib"]),
                     ],