Warn when users define system = true in sources

[achilleas-k]

Similar to #3435, we should warn when users include system = true in their source toml files that they add through composer-cli sources add source.toml. The system property is meant to indicate that an rpm repo is defined in the system-wide sources (in /usr/share/ or /etc/) instead of in the sources in the store when it's being printed through composer-cli sources info. Users sometimes use the output of the command to define their own source and define the value, expecting it to have an effect.

[ottobackwards]

It would be nice, if for each file type that could be uploaded, there was a verify call to check it.

[bcl]

It would be nice, if for each file type that could be uploaded, there was a verify call to check it.

They should just be checked when uploaded, as much as possible. The problem you can run into is that some failures can't be known until it is used to build an image. But we should add as much validation on the input as possible.