Allow ALIAS targets to alias IMPORTED targets.

This allows source compatibility such as making Qt4::Core an alias
of Qt5::Core during porting.

A future patch will allow deprecation of IMPORTED targets, so using
Qt4::Core in the above case would issue a message.
Merge topic 'allow-alias-imported-targets' into next

41c2c30 Allow ALIAS targets to alias IMPORTED targets.
Allow ALIAS targets to alias IMPORTED targets.

This allows source compatibility such as making Qt4::Core an alias
of Qt5::Core during porting.

A future patch will allow deprecation of targets, so using
Qt4::Core in the above case would issue a message.
Merge topic 'allow-alias-imported-targets' into next

dae2b18 Allow ALIAS targets to alias IMPORTED targets.