Fix macro hygiene about dataframesmeta.jl

[pdeffebach]

Here is the problem in particular:

julia> using DataFrames

julia> using DataFramesMeta: @transform

julia> df = DataFrame(x = [1, 2]);

julia> @transform df :y = :x .+ :x .^ 2
ERROR: UndefVarError: DataFramesMeta not defined
Stacktrace:
 [1] top-level scope
   @ ~/.julia/packages/DataFramesMeta/yzaoq/src/macros.jl:1190

The offending line is here.

Unfortunately I really don't know what to do in this exact instance, since this call is pretty deeply nested inside a helper function shared by everything. I can't just copy and paste it into each macro call. cc @jkrumbiegel , do you have any ideas on a fix?

[mcabbott]

The error from that line can probably be fixed by :($make_source_concrete($(Expr(:vect, t...)))), but I haven't tried.

[pdeffebach]

Cool that seems to have fixed it. Great, I was worried that I would be really out of luck. I will make a PR for this after I merge #307