fix convert package name with capital letters to rust file name

[rekby]

prost convert package name to snake while generate filename, but prost-wkt - not.

As result package "Sample.MyPackage" generate file "sample.my_package.rs" by prost. prost-wkt try to open it is "Sample.MyPackage.rs" and throw panic while open it

[fdeantoni]

Hi @rekby, many thanks for addressing this! I was wondering if, instead of copying the relevant functions, we could not just call the relevant function from prost-build instead? For example:

prost_build::Module::from_protobuf_package_name(package_name).to_file_name_or(package_name);

Would something like that be possible?

[rekby]

Done.

First time I was stopped by dependency version, now you updated dep version already, thanks.

Sorry, I can't check the fix in real project - in final I select pbjson-build as my dependency.