Reorg step2

[kt3k]

I tried the step 2 of #285. I moved things like the below:

//src/BUILD.gn -> //BUILD.gn
//src/.gn -> //.gn
//src/.gclient -> //.gclient
//src/deno.gni -> //deno.gni
//src/v8 -> //v8
//src/build -> //build
//src/build_overrides -> //build_overrides
//src/buildtools -> //buildtools
//src/testing -> //testing
//src/tools/clang -> //tools/clang
//src/third_party/googletest -> //third_party/googletest
//src/third_party/jinja2 -> //third_party/jinja2
//src/third_party/llvm-build -> //third_party/llvm-build
//src/third_party/markupsafe -> //third_party/markupsafe

I updated the build instructions as well.
I was able to build out/Debug/deno with this change.

[ry]

Thank you! This is very helpful and perfectly executed.

(ccache is not necessary, but I'll land that comment anyway)

[kt3k]

Thank you for landing! It's an honor 😃