find_dependency: Add namespacing for variables in the macro.

Avoid setting a variable called 'version' etc.
find_dependency: Ensure that ARGV1 has valid local content

The ARGV1 value could come from a wrapping context if find_dependency
is called from a function.
Merge topic 'fix-find_dependency-macro' into next

2be6dbe4 find_dependency: Ensure that ARGV1 has valid local content
a35ce723 find_dependency: Add namespacing for variables in the macro.