title: CMake find_package Boost fails on multiple usage
body: <p>I am trying to use a library that I wrote (is-msgs) that depends on boost::filesystem and have the following is-msgsConfig.cmake file:</p>
  
  <pre><code>include(CMakeFindDependencyMacro)
  
  find_dependency(Protobuf)
  find_dependency(spdlog)
  find_dependency(Boost COMPONENTS filesystem)
  
  include("${CMAKE_CURRENT_LIST_DIR}/is-msgsTargets.cmake")
  </code></pre>
  
  <p>If I try to use the library to build an executable, for instance, everything works like expected:</p>
  
  <pre><code>find_package(is-msgs)
  
  add_executable(example example.cpp)
  target_link_libraries(example is-msgs::is-msgs)
  </code></pre>
  
  <p>Although if my executable also depends on another boost component, for instance, boost::chrono the following code will not work:</p>
  
  <pre><code>find_package(Boost REQUIRED COMPONENTS chrono)
  find_package(is-msgs)
  
  add_executable(example example.cpp)
  target_link_libraries(example is-msgs::is-msgs boost::chrono)
  </code></pre>
  
  <p>Outputs:</p>
  
  <pre><code>CMake Error at CMakeLists.txt:11 (add_executable):
    Target "main" links to target "Boost::filesystem" but the target was not
    found.  Perhaps a find_package() call is missing for an IMPORTED target, or
    an ALIAS target is missing?
  </code></pre>
  
  <p>However, if I reorder the find_package instructions it works without a problem:</p>
  
  <pre><code>find_package(is-msgs)
  find_package(Boost REQUIRED COMPONENTS chrono)
  
  add_executable(example example.cpp)
  target_link_libraries(example is-msgs::is-msgs boost::chrono)
  </code></pre>
  
  <p>Is something wrong with my is-msgsConfig.cmake file or is this a bug in find_package(Boost) ? I am using CMake 3.11.1.</p>