Allow configuration of external module dependency locations

Teach the vtk_module_export_code_find_package macro to check for a
VTK_INSTALL_FIND_PACKAGE_<name>_DIR variable to configure the default
location of <name>_DIR to be reported to applications.  Allow the
variable to be set to a false value to disable the default.  This will
allow module dependencies on external packages such as Qt5 to be
configured for redistributable/relocatable packages.

Also allow applications to set <name>_DIR themselves before finding
VTK so that they can override the VTK-provided default location.

Change-Id: I0e452fb02f371636ec3a04954a99e33c7193f77a
Merge topic 'export-qt5-location' into master

ff7c659b Allow configuration of external module dependency locations