Rework embedded scripts to be loaded on-demand and by name

This will be required in order to migrate modules into the executable for binary releases

--HG--
branch : feature/improved-embedding
Added _MAIN_SCRIPT_DIR to module search path

--HG--
branch : feature/improved-embedding
Fix embedding of main script and manifest

--HG--
branch : feature/improved-embedding
Add more module and script search paths

Now uses same set of paths to locate both modules and individual scripts, in order:
  - the main script dir (_MAIN_SCRIPT_DIR)
  - the current working directory
  - ~/.premake
  - the --scripts=… argument, if present
  - the PREMAKE_PATH environment variable, if set
  - the folder containing the Premake executable (_PREMAKE_DIR)
  - ~/Library/Application Support/Premake
  - /usr/local/share/premake
  - /usr/share/premake

Prints a warning when embedded scripts are loaded from a debug build

--HG--
branch : feature/improved-embedding
Enable relative referencing of embedded scripts

--HG--
branch : feature/improved-embedding
Merge new script embedding system