Consider targets with double colons to be IMPORTED or ALIAS targets.

Introduce a policy to control the behavior.

The AliasTargets unit test already tests that using a
double-semicolon in the name is not an error. Change the ExportImport
test to use a namespace with a double-semicolon too.
Merge topic 'double-colon-is-imported' into next

36739a8 Consider targets with double colons to be IMPORTED or ALIAS targets.