Improved cmake build to build without to checkout "external/json" manually

[Bjoe]

This PR will fix the problem, if you would like to build these project against an already installed JSON "lib"/header.
Also I implemented a solution to "fetch" the sources without to fetch the sources "manually" into external/json

[codecov-commenter]

Codecov Report

Merging #5 (c11299d) into master (341b6ce) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   64.96%   64.97%   +0.01%     
==========================================
  Files         107      107              
  Lines        2189     2187       -2     
==========================================
- Hits         1422     1421       -1     
+ Misses        767      766       -1     

Impacted Files	Coverage Δ
include/tao/config/schema/internal/grammar.hpp	34.48% <0.00%> (-0.35%)	⬇️

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[Bjoe]

Unfortunately there is following in your JSON (also PEGTL) project:

In the JSON project you "alias" the JSON target from taocpp-json to ``taocpp::json. But when you "install" the lib/header and provide the cmake config-modul, the target is exported as taocpp::taocpp-json` because when you "export" the project it will be exported with the given namespace from `install(EXPORT ..... NAMESPACE ....)` in your solution it is `taocpp::` plus the target name, in your solution `taocpp-json`. The result is `taocpp::taocpp-json`