Consider using `pip freeze` when available

If pip is installed on the system executing dependency-check one will likely be able to get better dependency information by calling `pip freeze`.

See https://github.com/jeremylong/DependencyCheck/issues/963#issuecomment-341969087

Start by tracing how dependency-check gathers dependency information on the system executing it, then determine how pip availability and pip freeze output can be evaluated. Compare the proposed output with the existing dependency information; done means pip freeze is used when available and dependency results remain correct when it is not.

Written by the indexing model from the issue text.