When ngcc creates an entry-point program, the `allowJs` option is enabled
in order to operate on the JavaScript source files of the entry-point.
A side-effect of this approach is that external modules that don't ship
declaration files will also have their JavaScript source files loaded
into the program, as the `allowJs` flag allows for them to be imported.
This may pose an issue in certain edge cases, where ngcc would inadvertently
operate on these external modules. This can introduce all sorts of undesirable
behavior and incompatibilities, e.g. the reflection host that is selected for
the entry-point's format could be incompatible with that of the external
module's JavaScript bundles.
To avoid these kinds of issues, module resolution that would resolve to
a JavaScript file located outside of the package will instead be rejected,
as if the file would not exist. This would have been the behavior when
`allowJs` is set to false, which is the case in typical Angular compilations.
Fixes#37508
PR Close#37596
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