Add the ALIAS_LIBRARY target type.

* The ALIAS name must match a validity regex.
* Executables and libraries may be aliased.
* An ALIAS acts immutable. It can not be used as the lhs
  of target_link_libraries or other commands.
* An ALIAS does not appear in the generated buildsystem. It
  is kept separate from cmMakefile::Targets for that reason.
* A target may have multiple aliases.
* An ALIAS target may not itself have an alias.
* An IMPORTED target may not have an alias.
* An ALIAS may not be exported or imported.
Merge topic 'ALIAS-targets' into next

a5810fc Add the ALIAS_LIBRARY target type.