check_library_properties: Check if the files in the includes field exist

[per1234]

• This value could include a relative path to the file (e.g. includes=foobar.h,foo/bar.h) so I need to make sure that is correctly handled.
• External dependencies are sometimes added to the includes field but #include directives in sketches for dependencies of a library aren't necessary with any modern IDE version (and the includes field is only supported by modern IDE versions) so this is still a legitimate error.
  • The error message could mention this.