internal/postgres: move query to legacyUnitMetaQuery
internal/postgres: move query to legacyUnitMetaQuery

The query for GetUnitMeta is moved to legacyGetUnitMetaQuery, so that it
can be replaced in a future CL.

Change-Id: I45e56dc7c0d8c9a68c7f306ce203d50ef4a29196
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/276332
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite