[NFR]: Name the missing directory when config inference fails

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Goal: when crest cannot infer the project configuration, say which directory is missing.

With no crest.php, crest reads the psr-4 map in composer.json and uses the first entry whose directory exists. A project with a composer.json but no src/ fails with:

no crest.php and no usable psr-4 autoload entry found

The message does not say that a missing directory is the cause. Found by hand-testing the generators.

• Name the psr-4 directories that do not exist in the message (src/Project/Config.php, infer())
• A test asserts the new message

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Resolved in #24