Internal: Change snapshot state for unreleased versions and add validation tests for constants

[rjernst]

Currently the snapshot flag for Version constants is only set to true
for CURRENT. However, this means that the snapshot state changes from
branch to branch. Instead, snapshot should be "is this version
released?". This change also adds a validation test checking that
ID -> constant and vice versa are correct, and fixes one bug found there
(for an unreleased version).

[s1monw]

ok I think I like this. What I don't like is that we have nothing that detects if this info is accurate when we release. I think what we can do here is extend our release script to get all versions that are marked as snapshots and try to download them ie. check if there is a download for it on github or something like this? we can do this as a followup but I really want this info to be accurate. WDYT

[rjernst]

Sounds good, I created #9230.

[rjernst]

@s1monw Any other comments on this?

[jpountz]

Since the snapshot boolean is taking more importance, maybe we should not make it @Nullable in the constructor? Otherwise LGTM, thanks for opening this other issue about checking this flag when releasing.

[rjernst]

@jpountz Good point. I changed it from Boolean to boolean. The implication is interacting with an unknown version means it is now false instead of null. But places that check snapshot call snapshot() which returned false when it was null, so it shouldn't behave any different.

[jpountz]

LGTM