Do not mark imported libraries as GLOBAL in PCLConfig

[taketwo]

Fixes #2431.

Following up the discussion in #2431 I demoted exported targets to be local. The component "has been found already" test is reverted back to check PCL_xxx_FOUND variable. As a result:

• PCL found in sibling subdirectories are not visible to each other
• PCL found in parent directory is visible in subdirectories
• it's possible to find PCL several times in the same scope

A little more explanation for the last point. Suppose the root CMakeLists finds only few PCL components:

find_package(PCL REQUIRED COMPONENTS common features)

After that it's okay if in a subdirectory another search is performed with more components:

find_package(PCL REQUIRED COMPONENTS common visualization tracking)

@jasjuang Please give this an extensive testing :)

[jasjuang]

@taketwo I tested this with various different use cases of mine and it's all working. Please merge!