fix(composer): add Psl namespace mapped to src/Psl (#311)

Fixes #310

Tools such as `maglnet/ComposerRequireChecker` rely on symbols being
autoloadable, but due to the way the autoloader works here, it seems the
symbols can't be picked up. The issue does not affect `2.0.x-dev` since the
path `src/Psl` is mapped in `composer.json`, so the symbols are able to be
loaded. So this path map is backported to `1.9.x` series, so that tools that
work in this way can see the `Psl\*` functions.

composer.json

@@ -19,7 +19,8 @@
     },
     "autoload": {
         "psr-4": {
-            "Psl\\Integration\\": "integration"
+            "Psl\\Integration\\": "integration",
+            "Psl\\": "src/Psl"
         },
         "files": [
             "src/bootstrap.php"