Allow passing multiple files to determine sources from; The json output
is a dictionary of files to their metadata

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>

Move file processing into its own function; Will make it easier to add
support for multiple files later.

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>

Use the anyhow crate for error handler to remove a bunch of boilerplate
as well as better error reporting.

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>