Improve baseline basepath calculation

[frankdekker]

Type: Bugfix
Issue: fix #901
Breaking change: no

Previously the full filepath of the baselined files was calculated based on the location of the baseline xml file. This results in an issue when the baseline file was moved to a subfolder within the project.

Changes:

• No longer transform the filepaths within the baseline to a fullpath, but keep them relative.
• Changed the baseline matching if the violation file ends with the filename in the baseline.
• When generating the phpmd.baseline.xml, the basepath will be calculated based on the current working directory.
• Removed some Paths function that were no longer necessary.
• Updated coverage for the changes.
• Updated the documentation to reflect the change with the working directory.

Offtopic: \PHPMD\Renderer\SARIFRendererTest seems to fail on windows.