[vcpkg.cmake] check for vcpkg.json only in ${CMAKE_SOURCE_DIR}

[strega-nil]

So @Neumann-A has suggested this, and I think it's reasonable to do this. This means that we don't check outside of the directory containing the CMakeLists.txt.

I also have not changed the MSBuild, since I expect that many people will put their vcpkg.json into the solution directory, and there's not a standard variable for the solution directory. I'm not sure if this is the right thing to do; if anyone has opinions, please post them here.

Also, cc @Neumann-A of course.

[Neumann-A]

The question here is if you would like to support:

my-big-project/
  vcpkg.json
  CMakeLists.txt
  smaller-project/
    CMakeLists.txt 
    vcpkg.json
  other-project/
    CMakeLists.txt 
    vcpkg.json

which loads all vcpkg.json recursively. Instead of only the one from the toplevel project.
(needs a bit of investigation if each project() call reloads the toolchain)

[Neumann-A]

I also have not changed the MSBuild, since I expect that many people will put their vcpkg.json into the solution directory, and there's not a standard variable for the solution directory.

What is wrong with $(ProjectDir) ? (Nevermind you are already using MSBuildProjectDirectory which seems to resolve to the same directory.)

[strega-nil]

@Neumann-A solution != project.

The former would have to be done via interdependencies, and is made no different by this change.

[strega-nil]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[BillyONeal]

Thanks for your contribution :)