gxpkg improvements

[vyzo]

Drastically improves gxpkg:

• Directories:
  • update for the new directory format
  • add mighty-gerbils as the default directory
  • add a dir subcommand to manage the user directory, most notably add new directories saved in {GERBIL_PATH|~/.gerbil}/pkg/directory-list
• Dependency Management
  • adds a new deps subcommand to manage the dependencies of the current package/project
• Package Versioning with git tags
  • understands the @tag suffix in packages to mean "a git checkoutable thing"
  • implements (non strict) semver for choice of package when there is a version conflict. This is non strict in the sense that it always chooses the highest version, without putting a minor cap. We have discussed this behaviour with fare, belmarca and feeley and we concluded that it is better to put a subpackage v2 into your package once you are out of v1. Anyhow, I am happy with what we have now, but we can revisit in the future.
• Package build isolation and build manifests
  • each project is assembled in its own local GERBIL_PATH (unless the user has explicitly set GERBIL_PATH) to avoid interference from dirty ~/.gerbil.
  • each build has a manifest that record the exact dependencies at the time of build.

See #899
See #780
Closes #703
Closes #895
Closes #899

TBD:

• build isolation: Build Isolation and Manifests #918
• documentation: Document the new world order for builds #919
• more testing

[netlify]

✅ Deploy Preview for elastic-ritchie-8f47f9 ready!

Name	Link
🔨 Latest commit	fd8a7b5
🔍 Latest deploy log	https://app.netlify.com/sites/elastic-ritchie-8f47f9/deploys/65105dfd648c690008f25186
😎 Deploy Preview	https://deploy-preview-916--elastic-ritchie-8f47f9.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[fare]

do we need the v?

[vyzo]

yes, semver tags have a v before.

[vyzo]

let's follow go's convention here, it is good.