false positive in stale names with default value for function args

[ericphanson]

spotted since ExplicitImports says norm is stale in DataFrames, when it is not: https://github.com/JuliaData/DataFrames.jl/blob/027650418ab08bbe6b94f2cf42743839aa7a593e/src/abstractdataframe/abstractdataframe.jl#L524

Repro:

using LinearAlgebra: stale
f(norm=norm) = 1