Make `<source>` element in XML code coverage report optional

[sebastianbergmann]

Inspired by sebastianbergmann/php-code-coverage#1125 (comment), make <source> element in XML code coverage report optional to improve performance for use cases such as Infection's.

[sebastianbergmann]

This needs sebastianbergmann/php-code-coverage@c1888bb.

[staabm]

another data-point: while coverage-xml report generation was already super fast (~100ms) when running on the ParaTest codebase, adding --exclude-source-from-xml-coverage makes it ~50% faster

see paratestphp/paratest#1056