* allow environments to specify dev-build packages
* spack develop and spack undevelop commands
* never pull dev-build packges from bincache
* reinstall dev_specs when code has changed; reinstall dependents too
* preserve dev info paths and versions in concretization as special variant
* move install overwrite transaction into installer
* move dev-build argument handling to package.do_install
now that specs are dev-aware, package.do_install can add
necessary args (keep_stage=True, use_cache=False) to dev
builds. This simplifies driving logic in cmd and env._install
* allow 'any' as wildcard for variants
* spec: allow anonymous dependencies
raise an error when constraining by or normalizing an anonymous dep
refactor concretize_develop to remove dev_build variant
refactor tests to check for ^dev_path=any instead of +dev_build
* fix variant class hierarchy
7a6268593c