gxpkg improvements

[vyzo]

• [x] package versioning with tags; see also #703
• [ ] ability to use multiple packages within the same repo
• [x] build and clean command to support current directory building: #843
• [x] new command: gxpkg new to create package templates; this includes generating gerbil.pkg, build.ss, scaffolding for binary and library sources: #846
• [x] dep command: gxpkg dep [list|add|remove|update] ... to easily manage package dependencies in the current package.

[vyzo]

Moving to the backlog, as the multipackage repo thing doesn't have such a high impact.