Rename mkLFImported to importedIdLFInfo

The `mkLFImported` sounded too much like a constructor of sorts, when
really it got the `LFInfo` of an imported Id from its `lf_info` field
when this existed, and otherwise returned a conservative estimate of
that imported Id's LFInfo. This in contrast to functions such as
`mkLFReEntrant` which really are about constructing an `LFInfo`.