SI-8557 make scaladoc normalize paths of external jars.

Scaladoc compares (string representations of) the paths from
-doc-external-doc and the paths form `sym.underlyingSource`.
We now normalize on both ends before comparing them.
Merge pull request #3883 from gourlaysama/wip/t8557

SI-8557 make scaladoc normalize paths of external jars.