Expose command to build package metadata

[jaraco]

In pypa/packaging-problems#224 and #19, @Jonast and I are trying to work out how to use PEP 517 to query the package metadata for an unbuilt package. PEP 517 has an interface for prepare_metadata_for_build_wheel and a protocol for falling back to build_wheel to generate that metadata.

This package should present a command that will follow that protocol for producing such metadata from a source checkout, similar to what pep517.check does. It should probably present both as a command, leaving the metadata somewhere on the filesystem, and as an API - returning a path to the generated metadata.

[jaraco]

In pypa/packaging-problems#226, I propose an extension to the PEP 517 API that would enable tools like this package to build the package metadata and return a data stream or object in memory without having to rely on a file system.