Support `psr/container` v2.x

[mecha]

This package cannot currently be installed alongside psr/container v2.x. It explicitly depends on ^1.0.

Since v2 only adds the bool return type hint to ContainerInterface::has(), and given that this package doesn't have any classes that implement this interface, adding support should just be a matter of updating the version constraint in composer.json to also include ^2.0.

[mecha]

I started drafting a PR but I got stuck after finding out that the container-interop/service-provider package also depends on ^1.0 explicitly.

There is an open issue that discusses whether service-provider should depend on psr/container at all, and an open PR that removes the dependency. But until a solution gets merged into service-provider, this issue will likely remain blocked.

[XedinUnknown]

Oh! That's unfortunate. Yes, I understrand the problem. I'll try to do something about it soon. Sorry for the inconvenience!

[XedinUnknown]

Also. I'd like to re-visit the subject of removing dependency on service-provider altogether. Simply port the interface here. A bump to minor version of this package, and we move away. Since that standard appears unmaintained and nobody seems to be interested in it, I believe it's long overdue.

[mecha]

Cool. I'll go ahead and get started on a PR.

[mecha]

container-interop/service-provider now supports psr/container ^2.0 with the release of v0.4.1.