fix: ignore external modules

[tivac]

Description

Modules flagged using external were throwing a wobbily, so now we filter 'em out.

Motivation and Context

I've got a project that uses external and it was breaking.

How Has This Been Tested?

Added a test, also it doesn't break in my other project.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #555 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   99.03%   99.03%   +<.01%     
==========================================
  Files          46       46              
  Lines        1039     1040       +1     
  Branches      163      161       -2     
==========================================
+ Hits         1029     1030       +1     
  Misses         10       10

Impacted Files	Coverage Δ
packages/processor/processor.js	100% <100%> (ø)	⬆️
packages/rollup/rollup.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ffd29d9...a40e2a8. Read the comment docs.