feat: Add flag to exclude external targets

[honnix]

Closes #236

This PR adds a new flag --[no--]excludeExternalTargets that aims to support --noenable_workspace Bazel flag. This flag defaults to false so should be backward compatible.

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[tinder-maxwellelliott]

Thanks for doing this!

[meteorcloudy]

Thanks for the fix, can you make a new release?