Here is an idea that popped up in #236 and that builds on #232.

When the root package configures this:

 "extra": { "discovery": { "psr/http-factory-implementation": "My\\Psr17Factory" } }

then we shouldn't need auto-installing any extra packages for this virtual package.

Same when listing individual interfaces:

 "extra": { "discovery": { "Psr\\Http\\Client\\ClientInterface": "My\\Psr18Client" } }

In this case, we should verify that all the interfaces required to fulfill one of the supported *-implementation are listed.