DAPS 3.1 regression: If profiled-away XIncluded document does not exist, DAPS fails

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Problem description

DAPS 3.1 is profiling/validating differently than DAPS 3.0:
When validating a document profiled for os=sled, I am now seeing error messages for XIncludes that are included only in os=sles documents.
This is rather an issue in our translations, since we don't have symlinks for all those files we don't actually need in the document.

cf. https://travis-ci.org/github/SUSE/doc-sle/builds/771148858

This appears to be an issue with daps-xmlwellformed, assigning @tomschr .

Expected behavior

Profiled-away XIncludes are irrelevant and should not make DAPS fail.

Steps to reproduce problem

1. Go to doc-sle
2. Check out this commit off maintenance/SLE15SP2: git checkout 4ade29c86a
3. Go SLED/zh-cn: cd l10n/sled/zh-cn
4. Run DAPS validation with v3.0 and v3.1: rm -rf build && daps -d DC-SLED-all validate

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As a temporary workaround, I am now using this branch to build the OBS Docker container: https://github.com/openSUSE/daps/tree/bugfix-temp/3.1.1%2Bxmlwellformed-revert

Fwiw, the issue is gone when reverting this commit: 2656c7f (which is what I did in above branch).