Capitalize the folders inside src

[sanderdlm]

Lowercase folder names throw errors on Linux systems, which are case-sensitive.

According to the PSR-4 spec, the folder should match the sub-namespace: https://www.php-fig.org/psr/psr-4/#2-specification

The contiguous sub-namespace names after the "namespace prefix" correspond to a subdirectory within a "base directory", in which the namespace separators represent directory separators. The subdirectory name MUST match the case of the sub-namespace names.

The current package doesn't work with Composer on my Debian system.