txnbuild: Updates package with Protocol 14 changes.

[Shaptic]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Basically, I ran the following commands (simplified)

• rsync -c -r -u internal/txnbuild ../../
• find & replace "/services/horizon/internal/txnbuild" with "/txnbuild"
• rm -rf internal/txnbuild

All integration & unit tests pass.

Why

We want to make our Protocol 14 updates visible.

Known limitations

Because of the -u flag on the rsync command, I only copied new[er] files. This means that if someone updated txnbuild with a change after we updated the same file inside of internal/txnbuild, it wouldn't get copied. I don't think this is the case anywhere and tried confirming with diff, but a sanity check would be wonderful.

Questions:

• Do we need to update the CHANGELOG.md?
• Or make version bumps anywhere

[Shaptic]

Okay wait this is super incomplete, please don't review yet. was misreading the diff

I still think this is fine, as long as there were no changes to the same file in both txnbuild and internal/txnbuild. It looks like only #2967 was introduced during this time, and I've verified that those files still match.

[abuiles]

heads up, you are removing a bunch of valid files (they were removed from the internal txnbuild to make static check happy, but we shouldn't be removing files).

[2opremio]

@Shaptic If you want to be 100% sure, it would worth trying out https://stackoverflow.com/a/5190916 (in the example the use a directory from a remote repo, but I think it can be easily adapter to two directories in the same repo)