[Runtime] Fix using uninitialized variable "is_valid_" for WGT package
extracting.

The uninitialized variable "is_valid_" in package.cc(h) may lead to
error when installing a WGT package, and it should be initialized in
constructor.

Add a package type in Package class, since the WGT and XPK have
different extracting process, so need to add a flag to identify them.