improve top-level import speed with deferred imports

[sbailey]

This PR addresses several import speed issues identified in #2379, especially for desispec.io.

desispec.io: deferring imports of specutils, speclite.filters, and desispec.preproc improves "import desispec.io" from 4.1 seconds to 1.3 seconds (best of 5, pwd on scratch). The remaining time is dominated by importing astropy.table (1.0 sec), which would be messy to defer everywhere.

desispec.tsnr: deferring import astropy.convolution improves "import desispec.tsnr" from 2.9 to 2.3 sec. Remaining big imports are astropy.table and astropy.coordinates via desiutil.dust.

desispec.pixgroup: removing unused import healpix improves "import desispec.pixgroup" from 3.7 to 3.15 sec. Remaining big imports are desispec.tsnr (could be deferred, though this is a bit messy since TSNR is re-calculated on-the-fly if columns are missing from older data)

[coveralls]

coverage: 30.186% (-0.006%) from 30.192%
when pulling 5fabf40 on import_speed
into 05bcd76 on main.

[akremin]

Thanks, all these changes look good to me and I checked that there are no missing imports in any functions.

[sbailey]

For the record: I also ran a mini-prod with this branch in /global/cfs/cdirs/desi/users/sjbailey/spectro/redux/mini-pr2394 with jobs ccdcalib, arc, psfnight, flat, nightlyflat, tilenight, cumulative ztile, zpix, plus desi_tsnr_afterburner and desi_zcatalog. I think that covers everything touched by the deferred imports in this PR, e.g. to make sure that I didn't defer too much.