[BUGFIX] Fix looking up the default blueprint for scoped addons

[GendelfLugansk]

Fixes #10090 by checking if addon name starts with '@'

[bertdeblock]

Thanks for reporting the issue and making this PR.
I had a look at the PR that introduced this regression and I think we can actually avoid the sep + @ check if we write the lookup by path after the original lookup code:

Blueprint.lookup = function (name, options) {
  options = options || {};

- if (name.includes(path.sep)) {
-   let blueprintPath = path.resolve(name);
-   let isNameAPath = Boolean(blueprintPath);
-
-   if (isNameAPath) {
-     if (Blueprint._existsSync(blueprintPath)) {
-       return Blueprint.load(blueprintPath, options.blueprintOptions);
-     }
-
-     if (!options.ignoreMissing) {
-       throw new SilentError(`Unknown blueprint: ${name}`);
-     }
-
-     return;
-   }
- }

  let lookupPaths = generateLookupPaths(options.paths);

  let lookupPath;
  for (let i = 0; (lookupPath = lookupPaths[i]); i++) {
    let blueprintPath = path.resolve(lookupPath, name);

    if (Blueprint._existsSync(blueprintPath)) {
      return Blueprint.load(blueprintPath, options.blueprintOptions);
    }
  }

+ // Check if `name` itself is a path to a blueprint.
+ let blueprintPath = path.resolve(name);
+ if (Blueprint._existsSync(blueprintPath)) {
+   return Blueprint.load(blueprintPath, options.blueprintOptions);
+ }

  if (!options.ignoreMissing) {
    throw new SilentError(`Unknown blueprint: ${name}`);
  }
};

[bertdeblock]

I also noticed that the tests added in #9387 actually pass with and without the added code for supporting paths, so I'm going to try and fix that first to make sure we don't introduce any additional regressions.

[GendelfLugansk]

In this case, can we just add empty line to lookupPaths array? Inside generateLookupPaths, for example. But I am not sure this approach would not break support for specifying a path for the generate command.

[bertdeblock]

Made #10092 to fix the tests.

[kategengler]

Should we add a test for this case?

[bertdeblock]

#10092 was merged. Added a test for this fix as well.

[bertdeblock]

@GendelfLugansk Thanks for opening the issue and this PR!