Fix bad package names in generated documentation

[SeanTAllen]

Fixes incorrect qualified path name building for relative package imports,
Instead of doing the relative path in relation to the package we were in,
it did it based on the base of the program.

So for example instead of getting

"semver/version"
or an equiv
"semver/test/version/../../version"

You'd end up with

"semver/../../version"

[ponylang-main]

Hi @SeanTAllen,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3700.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.