Add import/export support and documentation for the SPDX_LICENSE
target property. A target's license can be specified by setting
this property to an SPDX license expression. CMake and CPS-format
export files generated with `export()` or `install()` will
retain the license information. CMake also imports the license
property for imported targets.
Closes: #26706
Rename the RunCMake/PackageInfo and RunCMake/CpsExportImport tests to
indicate that they cover CPS generated by the install command. This
opens space for similar tests that will cover CPS generation via the
export command.
Daniel Tierney