fix(@angular-devkit/build-angular): improve quality of localized source maps

[clydin]

Fixes: #17131

[alan-agius4]

Nice!

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     status "ci/angular: size" is failing
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[dgp1130]

Also merged to 9.1.x patch branch.

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