Rename composer.json.dist to composer.json

[mvorisek]

needed for #9241, for using this project/libs as a composer/Packagist dependency in general

[alecpl]

This will make working directly on source code more complicated. I normally have my composer.json file with additional packages/plugins added. I won't be able to do that. I need to investigate what options there are to fix that. One I found is use of COMPOSER env pointing to a different file.

[mvorisek]

COMPOSER env. var [1] is for root package [2] only even if the docs [1] do not mention that explicitly. Dependencies require to use composer.json strictly otherwise they cannot be installed. IFAIK there is no workaround and this PR is needed.

[1] https://getcomposer.org/doc/03-cli.md#composer
[2] https://getcomposer.org/doc/04-schema.md#root-package

[mvorisek]

This ensures the composer.json is downloaded still to composer.json-dist as expected by https://github.com/roundcube/roundcubemail/blob/1.6.5/bin/update.sh#L177 update script.

I couldn't find more places to fix, but please review this PR carefully, there might be more install/update usecases/docs I might be not fully aware.

[alecpl]

I created composer/composer#11784 but I don't expect it to be implemented soon or at all.

The package I mentioned there is not of a great quality. However, I tested the COMPOSER variable and it looks like it might actually work. But it's not an easy decision.

[mvorisek]

@alecpl can I ask you to review this PR? Based on #9279 (comment) and composer/composer#11784 the composer.json name is the only one name possible to make this project working with composer natively.

[mvorisek]

I have rebased this PR. @alecpl, can you please check? According to the sources above, there are no other options, the only supported filename by composer is composer.json and the place that needs to be adjusted is the distrubution/upgrade.