By getting rid of the PHONY install and bundling the go get with its uses we
can prevent unnecessary go gets in `make build` - this speeds up development as
I run `make generate-fixtures` a lot and fetching dependencies takes
comparatively long.
We also add the `-d` flag to go get to get rid of the deprecation warning
```
go get: installing executables with 'go get' in module mode is deprecated.
To adjust and download dependencies of the current module, use 'go get -d'.
To install using requirements of the current module, use 'go install'.
To install ignoring the current module, use 'go install' with a version,
like 'go install example.com/cmd@latest'.
For more information, see https://golang.org/doc/go-get-install-deprecation
or run 'go help get' or 'go help install'.
```
Until now we expected the .org file to print back to itself - we can't do that
when the input is not pretty printed already - with the introduction of blocks
with unindented content that will be the case.
including org files is more complex - e.g. footnotes need to be namespaced to
their source file. org does this by prefixing each included files footnotes
with a number - but even that is not enough as it doesn't guarantee
uniqueness.
As I don't have a usecase for it, I'll avoid the additional complexity for
now.
