Support explicit environments

[mivade]

Conda supports exporting explicit environments via conda list --explicit which results in output like the following (truncated for clarity):

# This file may be used to create an environment using:
# $ conda create --name <env> --file <this file>
# platform: linux-64
@EXPLICIT
https://repo.anaconda.com/pkgs/main/linux-64/_libgcc_mutex-0.1-main.tar.bz2
https://repo.anaconda.com/pkgs/main/linux-64/blas-1.0-mkl.tar.bz2
https://conda.anaconda.org/conda-forge/linux-64/ca-certificates-2019.6.16-hecc5488_0.tar.bz2
https://repo.anaconda.com/pkgs/main/linux-64/intel-openmp-2019.4-243.conda

This can be better for reproducibility since it lists the precise URLs for each package installed in an environment and also results in much faster environment creation since dependency resolution does not need to happen. Please consider a way to support environment creation from explicit package lists like this.