CMake Nightly Date Stamp
Only process transitive interface properties for valid target names.

Commit a1c4905f (Use the link information as a source of compile
definitions and includes., 2013-02-12) introduced the use of link
information as the source of target properties via the TARGET_PROPERTY
generator expression. This generator expression has a strict
interpretation of a valid target name and emits a fatal error for
invalid names.

Ensure that only targets with names valid for use with TARGET_PROPERTY
or targets which are determined by generator expressions are processed
by it. This means that at worst, invalid target names do not participate
in the transitive evaluation of properties, but the validation
generator expression can be extended where needed to resolve that.
Extend the range of valid target names with the + sign.

As noted in #13986, this character can commonly be used for target
names, such as those containing 'c++'.

Suggested-By: Benjamin Kloster
Merge topic 'fix-transitive-target-names' into next

eed2637 Extend the range of valid target names with the + sign.
254687d Only process transitive interface properties for valid target names.
efdf152 CMake Nightly Date Stamp
Mention that IMPORTED targets may be created by a find_package call.
Merge topic 'update-find_package-docs' into next

1922282 Mention that IMPORTED targets may be created by a find_package call.