Handle single sharded keyspaces for analysis

[dbussink]

This fixes the problem where we would throw an error for a query on a single unsharded keyspace, even though the limitation only exists for sharded queries.

The issue here was that the previous singleUnshardedKeyspace field was really the field indicating if we could short circuit analysis. schemadiff doesn't want to short cut full analysis though, since it does want to know table & column dependencies etc. which would not be computed if analysis can be shortcutted.

Since the singleUnshardedKeyspace field was overloaded and really was "can we shortcut" (either because of being in a single sharded keyspace or because of non-strict analysis being requested), we couldn't use that field as a fix to guard the error thrown.

Instead, we now have a proper explicit field canShortcut that we use to handle existing shortcut logic.

singleUnshardedKeyspace still exists, but now means what it actually implies, which is if this query runs against a single unsharded keyspace, irrespective of whether we can shortcut or not and is correctly set when doing full analysis.

With those changes, we can use singleUnshardedKeyspace to guard the error throw for unsupported sharded queries and schemadiff works properly as well.

Related Issue(s)

Fixes #16067

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[dbussink]

Changed this, since without this we'd segfault on checks after this if we have no error at all.

[systay]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.21%. Comparing base (2df3545) to head (fc286e9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16068   +/-   ##
=======================================
  Coverage   68.21%   68.21%           
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  Files        1541     1541           
  Lines      197330   197332    +2     
=======================================
+ Hits       134599   134604    +5     
+ Misses      62731    62728    -3     

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