Add flag to indicate only @NullMarked code should be checked

[msridhar]

Fixes #574

We add a new flag OnlyNullMarked to indicate that NullAway can only check code that is @NullMarked, and hence it is ok to omit the (previously required) AnnotatedPackages flag. We currently require exactly one of either AnnotatedPackages or OnlyNullMarked to be passed in; if both flags are omitted or both flags are passed, we fail.

As JSpecify and @NullMarked become more widely adopted, we may change the policy to allow neither flag to be passed (as NullAway already checks @NullMarked code out of the box). But for now, we require one of the flags to be passed, to avoid confusion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.17%. Comparing base (6d331c7) to head (210c4ad).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1117      +/-   ##
============================================
+ Coverage     88.14%   88.17%   +0.03%     
- Complexity     2234     2238       +4     
============================================
  Files            85       85              
  Lines          7237     7239       +2     
  Branches       1439     1439              
============================================
+ Hits           6379     6383       +4     
+ Misses          431      430       -1     
+ Partials        427      426       -1     

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