Add XDG Base Directory Specification support (#7386)

### Motivation:

As discussed in #6204, users would like to have a clean home directory.
And in the discussions following the merge of #3430, it seems there is a
willingness to adhere to the XDG spec, and support `~/.config/swiftpm`.

### Modifications:

If the `XDG_CONFIG_HOME` environmental variable is defined, use
`$XDG_CONFIG_HOME/swiftpm` as the root `dotSwiftPM` directory.

### Result:

The symlinks that were previously stored in `~/.swiftpm` are now stored
in `$XDG_CONFIG_HOME/swiftpm` when this variable is defined, therefore
not cluttering the home directory of users.

Closes #6204

---------

Co-authored-by: Max Desiatov <m_desiatov@apple.com>

Author: lordzsolt

Sources/Basics/FileSystem/FileSystem+Extensions.swift

@@ -211,9 +211,14 @@ extension FileSystem {
 
 extension FileSystem {
     /// SwiftPM directory under user's home directory (~/.swiftpm)
+    /// or under $XDG_CONFIG_HOME/swiftpm if the environmental variable is defined
     public var dotSwiftPM: AbsolutePath {
         get throws {
-            try self.homeDirectory.appending(".swiftpm")
+            if let configurationDirectory = EnvironmentVariables.process()["XDG_CONFIG_HOME"] {
+                return try AbsolutePath(validating: configurationDirectory).appending("swiftpm")
+            } else {
+                return try self.homeDirectory.appending(".swiftpm")
+            }
         }
     }