airframe-sql: Fix equi self-join

[NicolasRichard]

Certain queries containing equi self-join cannot be parsed:

select A.name
from A
inner join (select name from A) t2 
on A.name = t2.name

The type resolver will end up throwing an exception, complaining about ambiguous column references. This is because the source code tries to match columns based on their source table. I guess it's to handle joins with unqualified attributes.

It took me a while to understand what the code was doing. I think that the matcher should not look at the source table name if a qualifier is present.

[codecov]

Codecov Report

Merging #2819 (08920e4) into master (944fd01) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2819   +/-   ##
=======================================
  Coverage   82.45%   82.45%           
=======================================
  Files         337      337           
  Lines       14273    14273           
  Branches     2312     2310    -2     
=======================================
  Hits        11769    11769           
  Misses       2504     2504           

Impacted Files	Coverage Δ
...in/scala/wvlet/airframe/sql/model/Expression.scala	76.28% <100.00%> (ø)

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[takezoe]

Qualifiers were not considered. Makes sense!