XML: improve XML importer and document parser

[wenchy]

• fix: keep node's children order stable
• chore: remove deprecated XML test cases in test/dev

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Project coverage is 66.77%. Comparing base (2845eed) to head (2fb795e).

Files	Patch %	Lines
internal/importer/xml.go	92.85%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #117 +/- ## ======================================= Coverage 66.77% 66.77% ======================================= Files 44 44 Lines 5532 5532 ======================================= Hits 3694 3694 Misses 1357 1357 Partials 481 481 ```

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