Compiler: Add early error for output path being a directory.

- Introduce new `Path` type and an array of build paths on the build context.
- Resolve input and output paths/files early (before parsing).
- Error early if inputs are missing or outputs are directories.
- Plumb new file path generation into linker stage instead of its adhoc method.

TODO:
- Remove more adhoc file path generation in parser and linker stage.
- Make intermediate object file generation use new path system.
- Round out and robustify Path helper functions.

src/parser.cpp

@@ -5751,7 +5751,7 @@ ParseFileError parse_packages(Parser *p, String init_filename) {
 			}
 		}
 	}
-	
+
 
 	{ // Add these packages serially and then process them parallel
 		mutex_lock(&p->wait_mutex);