dtoc: Split source-code scanning to a separate file

Before expanding the scanning features any more, move this into a separate
file. This will make it easier to maintain in the future. In particular,
it reduces the size of dtb_platdata.py and allows us to add tests
specifically for scanning, without going through that file.

The pieces moved are the Driver class, the scanning code and the various
naming functions, since they mostly depend on the scanning results.

So far there is are no separate tests for src_scan. These will be added
as new functionality appears.

This introduces no functional change.

Signed-off-by: Simon Glass <sjg@chromium.org>