resolveExternalFields shouldn't search for fields

Spotted in: https://github.com/elastic/elastic-package/pull/840#discussion_r890943940

It doesn't seem to be a big issue if we consider only "integration" type packages, but it appears to be a problem when we consider "input" type packages. The current logic "knows" where fields can be found (`/fields` or `/data_stream//fields`) and it "globs" them there.

It seems to be a better idea if field locations are provided externally.

Start by locating resolveExternalFields and its callers, then trace how field paths are currently derived for integration and input packages. Define where callers should provide field locations and confirm that the existing package-root and data_stream paths are no longer searched implicitly.

Written by the indexing model from the issue text.