[Build/Dir] Reconstruct core source directory

[Build/Dir] Reconstruct core source directory

This commit reconstructs core source directory.

The below is the summary of what's changed.

1. reconstruct src directory (issue #153), like
+-- src/
|   +-- host/
|   +-- core/
|       +-- ne-*.c/ne-*.h
|       +-- ip/
|       +-- gem/
|       +-- npu/
|   +-- test/
|       +-- unittests/
|       +-- apptests/
2. change meson.build scripts to keep the same style.
3. change source file names to have the same prefix.

It resolves #153.

Signed-off-by: Dongju Chae <dongju.chae@samsung.com>