Write Go files with an internal helper instead of gofiles.Write

[tabboud]

Before this PR

A break (no more template rendering) and a behavior change (using packages.Load to get module paths instead of $GOPATH) in nmiyake/pkg/gofiles caused tests to fail on palantir/godel-okgo-asset-importalias#132 since we're no longer writing go.mod files.

All usages of gofiles.Write did not inspect the returned import paths, so instead of using that function, we can use an internal function to write out the raw Go files avoiding any package loading and only returning errors for writing the files.

This is almost identical to what was done for distgo here: palantir/distgo#237

After this PR

==COMMIT_MSG==
Write Go files with an internal helper instead of gofiles.Write
==COMMIT_MSG==

Possible downsides?

[nmiyake]

Based on the fact that this is only file that changes, it seems like this change alone doesn't remove the dependency for gofiles -- is that correct? It feels like this change would make more sense if it removed the gofiles dependency from this module (if not fully/transitively, then at least for direct reference)