Jsonnet Snippet Parser should translate params based on updated spec

This commit will cause the jsonnet snippet parser to parse input of the
form `import 'param://foo` to `params.foo` as opposed to the previous
`${foo}`.

${foo} does not provide an easily accessible way to the params that are
exposed through the params.libsonnet file. Since parameters are being
exposed using a local 'params' variable, it makes sense for the jsonnet
snippet to parse `import param://foo as `params.foo` instead.