Reorganize Go coder packages.

Context is in the comments here: https://github.com/apache/beam/pull/12059

"As an aside, and not in this PR, what do you think about refactoring the locations of all the coder code in the SDK right now. I noticed while trying to figure out how to make some coders that its very unintuitive trying to figure out how to make coders for certain types (for example ints). Right now it seems like built-in coders are all in graph/coder and custom coders for Go types are in runtime/coderx."

This would probably involve moving code from coderx into a more intuitive directory/package. Maybe something like graph/coder/custom, putting it as a subdirectory to the built-in coders. This would almost definitely need to wait until a major version update so we don't impact users that are using these packages already.

Imported from Jira [BEAM-10309](https://issues.apache.org/jira/browse/BEAM-10309). Original Jira may contain additional context.
Reported by: danoliveira.