Run Spago from a nested directory other than workspace root

[fsoikin]

Description of the change

Fixes #1237
Allowing Spago to be launched from a directory nested within the workspace.

• When launched, Spago will walk directories up until it finds a spago.yaml containing a workspace.
• As it goes, it remembers config docs it finds and loads along the way.
  • We have to load them anyway in order to see if they contain a workspace. So we remember them in order to avoid loading them twice.
• After finding the root, it then globs for spago.yaml files back down the tree and loads those that it hasn't loaded while walking up.
  • The subprojects from down the tree are loaded in the order top-down, such that parent directories are traversed before their children.
  • If Spago encounters a config with a workspace, it remembers its path and does not attempt to load any configs nested under it.
  • If Spago encounters an unreadable config (e.g. malformed or with unknown fields), it will not attempt to load any configs nested under it.
• As it walks up the tree, it also gathers and remembers any spago.yml (wrong extension) files.
• As it walks up the tree, it also remembers the first spago.yaml it sees. If this is not the root project, it will become "selected".
  • I'm not 100% on this. Previous behavior has been to never "select" a project unless there is a -p option. But on the other hand, it's very likely that if your CWD is a particular project, you're probably working on just that one.
• If it goes all the way up to the file system root, it prints an error saying no config found and lists all the yml files found along the way, if any, suggesting that they may have the wrong extension.

Checklist:

• Added the change to the "Unreleased" section of the changelog
• [ ] Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

[fsoikin]

All feedback addressed except the request to mention #951, because I didn't understand what it meant.

[peterbecich]

Thank you @fsoikin for implementing this feature!