imp: pivot: Tag name shadowing field name takes precedence (#2169)

[savanto]

Please see #2169 for the rationale behind this change.

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Tags shadowing field names (eg. payee) take precedence during pivoting. For example, before this change,

2025-01-01 CustA
    asset       $100    ; payee:CustA
    asset       $200    ; payee:CustB
    revenue

we expect bal asset --pivot=payee to produce:

                $300  CustA
--------------------
                $300

But after this change, we expect:

                $100  CustA
                $200  CustB
--------------------
                $300

More specific tags take precedence over less specific tags (ie. posting tags take precedence over transaction tags):

2025-01-01 CustA  ; payee:CustB
    asset       $100    ; payee:CustA
    asset       $200    ; payee:CustB
    revenue

still results in

                $100  CustA
                $200  CustB
--------------------
                $300