Add prefix to classpath of generated protobuf files

[jeromesklar-toast]

Adds a protokt specific prefix to classpath of generated protobuf files to avoid conflicts in projects that use both protokt and another protobuf library, such as that of google. This will allow projects to safely use protokt and another protobuf library and not have conflicts (which can lead to RuntimeExceptions) between generated file paths.

Note: this is a breaking change and upgrading to this version of protokt will involve making changes to add the "protokt." to certain imports

Testing: Verified this fixes at least one runtime exception that was occurring with google protobuf and protokt libraries in the same service

Future changes:
In the future, we plan on potentially doing some more package cleanup to:

• remove some extraneous "com." prefixes
• remove the special case that converts com.google.protobuf --> com.toasttab.protokt and just universally add the "protokt." prefix solution in all cases. (so, com.google.protobuf --> protokt.com.google.protobuf)

[andrewparmet]

Looks great.

[andrewparmet]

Two tiny things and then this is ready to merge.

[ogolberg]

Let's punt till 1.0.

• The cost of propagating this backwards-incompatible change through systems that use protokt is substantial.
• Current behavior is consistent with Wire and ScalaPB (other protobuf codegens that target JVM languages)

[andrewparmet]

superseded by #168