find_package: Add support for default GLOBAL imported targets

Allow find package to promote scope of imported targets by specifying
an argument to `find_package` or by specifying a CMake variable.
    * Add support for CMAKE_GLOBAL_IMPORT_SCOPE variable
    * Add support for GLOBAL argument to find_package

Additionally add testing for above features.

Help/command/find_package.rst

@@ -79,7 +79,8 @@ Basic Signature
   find_package(<PackageName> [version] [EXACT] [QUIET] [MODULE]
                [REQUIRED] [[COMPONENTS] [components...]]
                [OPTIONAL_COMPONENTS components...]
-               [NO_POLICY_SCOPE])
+               [NO_POLICY_SCOPE]
+               [GLOBAL])
 
 The basic signature is supported by both Module and Config modes.
 The ``MODULE`` keyword implies that only Module mode can be used to find
@@ -115,6 +116,11 @@ define what occurs in such cases.  Common arrangements include assuming it
 should find all components, no components or some well-defined subset of the
 available components.
 
+Specifying the ``GLOBAL`` keyword will promote all imported targets to
+a global scope in the importing project. Alternatively this functionality
+can be enabled by setting the variable
+:variable:`CMAKE_FIND_PACKAGE_TARGETS_GLOBAL`
+
 .. _FIND_PACKAGE_VERSION_FORMAT:
 
 The ``[version]`` argument requests a version with which the package found