Allow specifying path for the `common` module from the outside

[bugadani]

This enables chiptool generate --svd atsaml10d14a.svd --common-module super::common and same for gen-block

[Dirbaio]

passing super::common won't work in the general case when you generate things at multiple depths in the module tree. To do it right you'd have to go through relative_path somehow.

[bugadani]

umm that sounds way too much a hassle for this, I guess my example should have been crate::pac::common :)

The hard part is that my use case that I wanted to support here is essentially to cut-and-paste parts of the result into my project, so the module which references common doesn't have a predefined path in chiptool.