replace eslint-import-resolver-alias with eslint-import-resolver-custom-alias

Summary: replace eslint-import-resolver-alias, which has been abandoned, with eslint-import-resolver-custom-alias, which is somewhat more actively maintained.

Details: eslint-import-resolver-alias has been abandoned; eslint-import-resolver-custom-alias is somewhat more actively maintained. An open issue reports that aliases will not properly resolve with eslint-plugin-import >= 2.20.1, which we use and which is required by newer versions of eslint-config-airbnb.

We will have to decide if this constitutes a major change or not. Technically, it will be necessary to update dependent repositories' .eslintrc files from

to