fix: use -m flag for apidiff to signal that we are operating in a module

[arunpoudel]

Looks like apidiff was only operating in package level. Which meant that only packages that were included by files in root directory were enumurated when considering the apidiff.

Treating the folder as a module, enumurates all path and sub-path. See on discussion related to this.

Fixes: #12

[bendiknesbo]

I didn't notice that you also created a PR, so I also created a PR #14. Your PR is missing the -m flag on the comparing-command, so it will fail with an error like:

➜  8:44:35 go-datadog-lib git:(main) ✗ apidiff old.txt new.txt 
reading export data from old.txt: unexpected export data with prefix "\x00\x02\xef\x8c\x03\xa2\xa9\x06-g" for path github.com/coopnorge/go-datadog-lib/v2

[arunpoudel]

Resolved by #14