Generate command doesn't properly handle relative profile imports

[anweiss]

When executing oscalkit generate and processing profile import's, relative paths are not being taken into account properly. We're incorrectly taking the path relative to the directory from which the oscalkit CLI is being run. However, we instead should be taking the path relative to the directory in which the profile is stored. The implementation of the following function looks to be the culprit:

https://github.com/docker/oscalkit/blob/d66e30b99ca1454f7dfd40ba5982bd7ed8be8ef2/generator/reader.go#L49

So for example, if I execute oscalkit generate -p /someDir/profile.xml and profile.xml has a relative import like ../catalog.xml, then the directory that needs to be searched is /someDir and not the directory in which the oscalkit generate command is being run.

This supports #18 and #12

[minhaj10p]

@anweiss
issue addressed in #58 which is dependent upon #52

[minhaj10p]

@anweiss We can close this issue once #66 gets merged.

[anweiss]

awesome, thanks @minhaj10p