Feature request: export lockfile in SPDX format

[jwarwick-delfi]

As a consumer of Pex lockfiles via the pants build tool, I would like to export a lockfile in an open format that I can use to generate a software bill of materials (SBOM). SPDX seems to be the widely-used open standard for these files.

SPDX can be expressed in a variety of formats, personally I would prefer text, JSON, or YAML.