Fix up parsoidsvc repositories location

The parsoidsvc-php-parsertests depends on two repositories:

- mediawiki/services/parsoid which is fetched with the Jenkins git
  plugin in $WORKSPACE and is instructed to clear it out
- mediawiki/core which is fetched via git-archive in the same location
  and hence is cleared out by Jenkins git plugin

To ease debugging, fetch mediawiki/core to a different directory and
manually clean it up using the good old rm -fR

Parsoid is now fetched under /parsoidsvc

mediawiki/core is fetched under src/mediawiki/core. The mw-set-env.sh
env script detects that and set MW_INSTALL_PATH accordingly.

Change-Id: I51771f6900a672bffe397283925a9b2236473d35