Compute join on expressions immediately, rather than after views have been translated

[christopherswenson]

This fixes an issue where views that used joins which had composite field usage in their on expressions would incorrectly not take that composite field into account when resolving a cube.

source: extended is compose(
  base,
  base extend { dimension: composite_field is 1 }
) extend {
  join_xxx: `join` is ... on composite_field = ...
  view: `view` is { group_by: `join`.field }
}
run: extended -> `view`

Previously, we were storing all the joins while computing fields of a source (including views), then computing all the join on expressions after translating all the fields. Now we compute the on expressions immediately after adding each join.