feat: dependency graph with virtual packages

[jkowalleck]

how is the dependency graph with "virtual packages" or dependency to capabilities?
see https://getcomposer.org/doc/04-schema.md#provide

these relations should result in a relation in the dependency graph.

example:
package A depends on psr/log-implementation
package B depends on A and depends on cakephp/log (which provides psr/log-implementation)

this means , that, effectively, a graph could look like:

[
  {
    ref: 'A',
    dependsOn: ['psr/log-implementation'],
  },
  {
    ref: 'B',
    dependsOn: ['A', 'cakephp/log'],
  },
  {
    ref: 'psr/log-implementation',
    dependsOn: ['cakephp/log'],
  },
  {
    ref: 'cakephp/log'
  },
]