Expand git.get_repo_metadata() to get repo metadata from installed packages

[artgoldberg]

Data files need to store the versions of the schema and data packages on which they depend. git.get_repo_metadata() obtains these versions from local git repos being used to create a data file.

Expand this to obtain version info from pip-installed instances of a package being used to create a data file. the package must contain a unique identifier that can be associated with a repo reference, such as a tag.

Possible approach

Prepare the package

• Finalize a package release
• commit it
• tag it
• write the tag into the release
• commit that
• push to git repo
• install on PyPI

Get version from the package

• read tag
• get package repo URL from pip show 'Home-page' line
• clone the repo
• use "git show-ref -s tag" to get the package version