Fix –-toolchain value shadowed by local executables (#7483)

Behavior of `--toolchain` regressed in
#7296 by
preferring the first match in toolset root paths: when `swiftc` is
available in the same directory next to the SwiftPM binary being
invoked, that became preferred with `--toolchain` value ignored.

This is now fixed by slightly tweaking where custom toolchain gets added
- it's prepended.

A newly added test verifies that the behavior is fixed. For that we had
to inject `FileSystem` and `EnvironmentVariables` in a lot more places,
with an added benefit of making our tests more consistent and isolated
from an arbitrary build environment.

Resolves: rdar://126095653

---------

Co-authored-by: Pavel Yaskevich <pyaskevich@apple.com>
(cherry picked from commit ee9d0b2)

# Conflicts:
#	Sources/PackageModel/UserToolchain.swift

Author: MaxDesiatov

Sources/Basics/CMakeLists.txt

@@ -29,6 +29,7 @@ add_library(Basics
   Errors.swift
   FileSystem/AbsolutePath.swift
   FileSystem/FileSystem+Extensions.swift
+  FileSystem/InMemoryFileSystem.swift
   FileSystem/NativePathExtensions.swift
   FileSystem/RelativePath.swift
   FileSystem/TemporaryFile.swift

Sources/Basics/FileSystem/FileSystem+Extensions.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift open source project
 //
-// Copyright (c) 2020-2021 Apple Inc. and the Swift project authors
+// Copyright (c) 2020-2024 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See http://swift.org/LICENSE.txt for license information