`Conda.search` is broken

[eschnett]

I say

using Conda
Conda.search("asdf")

and receive the error message

julia> Conda.search("asdf")
ERROR: MethodError: Conda.search(::String, ::String) is ambiguous. Candidates:
  search(package::AbstractString, _ver::Union{VersionNumber, AbstractString}) in Conda at /Users/eschnett/.julia/packages/Conda/m7vem/src/Conda.jl:235
  search(package::AbstractString, env::Union{AbstractString, Symbol}) in Conda at /Users/eschnett/.julia/packages/Conda/m7vem/src/Conda.jl:230
Possible fix, define
  search(::AbstractString, ::AbstractString)
Stacktrace:
 [1] search(::String) at /Users/eschnett/.julia/packages/Conda/m7vem/src/Conda.jl:230
 [2] top-level scope at none:0

This is with Julia 1.0 and Conda 1.0.1 on macOS.

[stevengj]

We should probably switch to keyword args.