[heft-sass-plugin] Upgrade sass-embedded and fix indirect dependency resolution in import and use rules.

[iclanton]

Summary

This PR bumps sass-embedded to make use of a new parameter provided to the findFileUrl callback. This parameter provides the context from which a file is being resolved, allowing a correct resolution algorithm to be employed. This PR also implements that algorithm. With this change, @import and @use rules in dependency scss files work correctly, even if the project being built doesn't have a direct dependency on the package that the rule resolves to.

How it was tested

Tested in a large repo that previously had this issue.

Impacted documentation

None.