Repository file format: group attributes of a package

* Introduce new package attribute B_HPKG_ATTRIBUTE_ID_PACKAGE (valid
only in a repository file) to group the attributes belonging to a
package.
* BRepositoryContentHandler:
- No longer derive from BPackageContentHandler.
- Add hooks HandlePackage() and HandlePackageDone() that bracket the
attributes for a package. This is more explicit and robust than
handlers having to guess when one package ended and the next began.
* BRepositoryCache: Make use of BPackageInfoContentHandler. No need to
duplicate the code for reading a package info from package info
attributes.

Repository file format: group attributes of a package

* Introduce new package attribute B_HPKG_ATTRIBUTE_ID_PACKAGE (valid
only in a repository file) to group the attributes belonging to a
package.
* BRepositoryContentHandler:
- No longer derive from BPackageContentHandler.
- Add hooks HandlePackage() and HandlePackageDone() that bracket the
attributes for a package. This is more explicit and robust than
handlers having to guess when one package ended and the next began.
* BRepositoryCache: Make use of BPackageInfoContentHandler. No need to
duplicate the code for reading a package info from package info
attributes.