Fix destination path to be relative to current directory.

[vecchiot-aws]

Issue #, if available:

Fixes issue #45

Description of changes:

Converts to/from files to relative paths before passing them into path_to_file, which expects two paths relative to the same root.

I don't believe this should cause issues with existing CBMC-viewer usage:

• If both paths are already relative (even if not relative to current directory) , then os.path.relpath won't change either of them, so there is no issue.
• If both paths are absolute, then they still have the same relative difference, and so path_to_file should return the same value.
• If one of the paths is absolute and the other is relative to the current directory, then os.path.relpath will not change the relative difference between the two paths.
• If one of the paths is absolute and the other is relative to a directory other than the current directory, then this change will lead to invalid links, however this scenario would already lead to invalid links prior to this PR.

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[markrtuttle]

I prefer this solution https://github.com/awslabs/aws-viewer-for-cbmc/pull/47

[markrtuttle]

Closing in favor of https://github.com/awslabs/aws-viewer-for-cbmc/pull/47