Fixing DiktatRuleSet crashes when there are no package and no imports

[petertrr]

What's done:

• Fixed logic in rules
• Added tests
• Added way to disable several rules in smoke test
• FileStructureRule works when package and/or imports are missing

[codecov]

Codecov Report

Merging #464 into master will decrease coverage by 0.12%.
The diff coverage is 48.78%.

@@             Coverage Diff              @@
##             master     #464      +/-   ##
============================================
- Coverage     81.87%   81.74%   -0.13%     
- Complexity     1478     1499      +21     
============================================
  Files            71       71              
  Lines          3730     3731       +1     
  Branches       1195     1203       +8     
============================================
- Hits           3054     3050       -4     
- Misses          203      209       +6     
+ Partials        473      472       -1     

Flag	Coverage Δ	Complexity Δ
#unittests	81.74% <48.78%> (-0.13%)	1499.00 <26.00> (+21.00)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...tlin/org/cqfn/diktat/ruleset/utils/AstNodeUtils.kt	82.54% <33.33%> (+0.38%)	0.00 <0.00> (ø)
...fn/diktat/ruleset/rules/files/FileStructureRule.kt	69.09% <48.00%> (-5.44%)	46.00 <18.00> (+14.00)	⬇️
...fn/diktat/ruleset/rules/kdoc/CommentsFormatting.kt	68.42% <53.84%> (-1.21%)	69.00 <8.00> (+5.00)	⬇️
...tlin/org/cqfn/diktat/ruleset/constants/Warnings.kt	98.19% <0.00%> (+0.90%)	10.00% <0.00%> (+1.00%)
...fn/diktat/ruleset/rules/classes/DataClassesRule.kt	86.84% <0.00%> (+2.63%)	26.00% <0.00%> (+1.00%)

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[orchestr7]

lgtm