WIP: Generate Spack resource statements

[hartzell]

This is a work in progress, not ready to be merged. It might change with feedback from @dmgk and/or the Spack community.

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This adds support for generating Spack resource statements, in support of adding general support for Go applications to Spack. It:

• adds two command line switches (-spack and -app_version)
• prints Spack resource statements to stdout when -spack is provided on the command line, including a when=... attribute if -app_version is provided.
• includes tests for general functionality, the inclusion of the when= attribute, and differentiating between the commit= and tag= commit depending on the format of the version identifier.

Closes #7

[dmgk]

LGTM, please let me know when it'll be ready to be merged.

[hartzell]

By way of an update, the Spack PR isn't getting timely feedback, so this is on hold until it gets moving again. Thank you for being willing to see the use cases expanded!