minor #16397 added the new Composer exclude-from-classmap option (annesosensio)

This PR was merged into the 2.3 branch.

Discussion
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added the new Composer exclude-from-classmap option

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
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65bef75 added the new Composer exclude-from-classmap option

Author: fabpot

composer.json

@@ -33,7 +33,10 @@
         "symfony/http-foundation": ""
     },
     "autoload": {
-        "psr-0": { "Symfony\\Component\\Form\\": "" }
+        "psr-0": { "Symfony\\Component\\Form\\": "" },
+        "exclude-from-classmap": [
+            "/Tests/"
+        ]
     },
     "target-dir": "Symfony/Component/Form",
     "minimum-stability": "dev",