Avoid the timestamp check for `add-source --snapshot` dependencies.

Snapshot deps now behave more like `cabal-dev add-source`. Implemented by adding
a BuildTreeRefType field to the BuildRef index entry.

Fixes #1321.
Merge pull request #1322 from 23Skidoo/snapshots-avoid-timestamp-check

Avoid the timestamp check for `add-source --snapshot` dependencies.