contrib/schema: update generator to rewrite imports

The generator is a bit weird; it insists on writing this redundant `schema-api.json` file with a non-configurable name and in a strange path based on GOPATH ([1], [2]), which a while ago resulted in this file being created. It's totally redundant with the input JSON file and so there's no point in keeping it in the repo. I changed the schema-generator script to set GOPATH to the ephemeral gopath/ directory instead so this file just gets written there.

[1] https://github.com/coreos/fleet/pull/777/files#diff-bcc28916b4bf2d8a0e4175805d9497aeR345
[2] https://github.com/coreos/fleet/pull/777/files#diff-bcc28916b4bf2d8a0e4175805d9497aeR278