[Webpack 5 Regression] TerriaMap now requires terriajs devDependencies

[pjonsson]

@na9da #726 passed the CI before this change (but after the ESLint update), but I just rebased it and it now fails:

ERROR in ./node_modules/terriajs/test/Models/Catalog/CatalogGroups/OpenDataSoftCatalogGroupSpec.ts:6:23
TS2307: Cannot find module 'fetch-mock' or its corresponding type declarations.
    4 | import OpenDataSoftCatalogItem from "../../../../lib/Models/Catalog/CatalogItems/OpenDataSoftCatalogItem";
    5 | import Terria from "../../../../lib/Models/Terria";
  > 6 | import fetchMock from "fetch-mock";
      |                       ^^^^^^^^^^^^
    7 |
    8 | import facets from "../../../../wwwroot/test/ods/facets.json";
    9 | import datasets from "../../../../wwwroot/test/ods/datasets.json";

The difference is from commit 28aa111 to commit 10c2192.

Originally posted by @pjonsson in #718 (comment)

It seems the test files have been included in the terriajs-package before, so I'm guessing something changed with the updated webpack config.