Allowing a derived variable to reuse a physical column's name was wrong: an
intra-row calculation (SUMMER = SUMMER + 1) implies a retained value, which
depends on the order rows are read. The same argument rules out a variable
listing itself as a dependency.
Reverted:
- the name collision is an error again ('VAR_NM collides with a column in the
target table')
- VAR_DEPS may not list the variable itself
- the physical column list is back to mf_getvarlist - the type/length check
only existed to support shadowing
- the droplist guards in stagedata / viewdata / getrawdata, and the duplicate
suppression in getdata, are no longer needed
- the docs and both test suites drop the shadowing case
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