Conda Package parsing brittle when `build` contains an underscore

[madpah]

Raised in #51, an example of a valid Conda package URL (https://conda.anaconda.org/conda-forge/linux-64/_libgcc_mutex-0.1-conda_forge.tar.bz2#d7c89558ba9fa0495403155b64376d81) has been provided which breaks our parsing logic.

Specification for Conda packages is here, but this doesn't seem to be deterministic in all cases unfortunately.