[rql2sql] Handle comparison of eid on variables from an subquery
We don't want to force using 'identity' in this case which may be easily
handled by detecting variable is a `ColumnAlias` and relation is 'eid'.
Closes #15393583
^build$
^dist$
\.egg-info$
^.tox$
^.cache$
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\.swp$
^doc/book/en/apidoc$
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syntax: regexp
.*/data.*/database/.*
.*/data/ldapdb/.*
.*/data/uicache/
.*/data/libpython/cubicweb_.*/i18n/.*\.po
^doc/html/
^doc/doctrees/
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test-results.xml