[themes] Component shadowing doesn't resolve properly if multiple themes shadow the same component

[jlengstorf]

Description

In a theme scenario where there are multiple ancestors, component shadowing appears to only respect the first component (or it stops looking after a certain point).

Example (site uses theme3, theme3 uses theme2, theme2 uses theme1):

=> theme1/src/components/foo.js
=> theme2/src/theme1/components/foo.js (actually shows up)
=> theme3/src/theme1/components/foo.js (should show up)
=> site (where develop is run from)

Steps to reproduce

See this repo: https://github.com/jlengstorf/theme-shadowing-bug

Environment

  System:
    OS: macOS 10.14.3
    CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 10.14.2 - ~/.nvm/versions/node/v10.14.2/bin/node
    Yarn: 1.13.0 - /usr/local/bin/yarn
    npm: 6.4.1 - ~/.nvm/versions/node/v10.14.2/bin/npm
  Languages:
    Python: 2.7.15 - /usr/local/bin/python
  Browsers:
    Chrome: 72.0.3626.109
    Firefox: 63.0
    Safari: 12.0.3