Separate internal macro into KDevPlatformMacrosInternal.cmake

Summary:
Also

• add dox for KDEVPLATFORM_ADD_PLUGIN
• revert "Do not use $<BUILD_INTERFACE> in installed KDevPlatformMacros.cmake"

Motivation for e32618d64ac4fadbdceb228326df63cfac4b8bc8 was that
kdevplatform_add_library seemed part of public macross, so same like
kdevplatform_add_plugin, due to being in installed & exported macros file.
Though with KDevPlatform_SOURCE_DIR & KDevPlatform_BINARY_DIR not
defined when the macro is imported in other projects, such $<BUILD_INTERFACE>
would be bogus.

Looking again, KDEVPLATFORM_VERSION & KDEVPLATFORM_LIB_SOVERSION would
also not be defined. And actually this macro might not be meant for
public consumption, given that a platform usually is defined by
a given set of libs, so 3rd-party libs should not want to use this macro
anyway. So moving the macro out of the installed macro file makes this
clear and prevents abuse.

Reviewers: KDevelop, apol, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D7217