Deprecate package in favor of composer-runtime-api

[Ocramius]

See composer/composer#8803

We can likely drop most of the contents of this library if we pin to composer/composer:^2.

After that, the library can be marked as security-only, and PackageVersions\Versions no longer needs to be generated, but rather relies on composer's built-in mechanisms.

Supersedes #135

[Ocramius]

This can be done in 2.0.0, once we can get rid of PackageVersions::ROOT_PACKAGE_NAME completely

[Ocramius]

Potentially to be done in 2.1.0 or later.

[Ocramius]

Decided that package is not to be deprecated: composer-runtime-api:^2 seems to break regularly, so having this package to at least test it regularly is not a bad idea.

In addition to that, this package at least provides a @psalm -friendly API exposing purity markers and guarantees on return types.