PackageModel: make Toolset.init(toolchainBinDir:buildFlags:) public (#7064)

Previously the only available initializer allowed creating toolsets from
serialized form stored on a file system. This initializer allows
creating `Toolset` values directly with given build flags and toolchain
binaries directory path, which may be needed by libSwiftPM clients.

Author: MaxDesiatov

Sources/PackageModel/Toolset.swift

@@ -157,7 +157,11 @@ extension Toolset {
         }
     }
 
-    init(toolchainBinDir: AbsolutePath, buildFlags: BuildFlags) {
+    /// Initialize a new ad-hoc toolset that wasn't previously serialized, but created in memory.
+    /// - Parameters:
+    ///   - toolchainBinDir: absolute path to the toolchain binaries directory, which are used in this toolset.
+    ///   - buildFlags: flags provided to each tool as CLI options.
+    public init(toolchainBinDir: AbsolutePath, buildFlags: BuildFlags = .init()) {
         self.rootPaths = [toolchainBinDir]
         self.knownTools = [
             .cCompiler: .init(extraCLIOptions: buildFlags.cCompilerFlags),