fix(router): support lazy loading for empty path named outlets

[atscott]

In general, the router only matches and loads a single Route config tree. However,
named outlets with empty paths are a special case where the router can
and should actually match two different Routes and ensure that the
modules are loaded for each match.

This change updates the "ApplyRedirects" stage to ensure that named
outlets with empty paths finish loading their configs before proceeding
to the next stage in the routing pipe. This is necessary because if the
named outlet has loadChildren but the associated lazy config is not loaded
before following stages attempt to match and activate relevant Routes,
an error will occur.

fixes #12842

global presubmit - Will want to do another rerun as I made a tiny change since then to pass the test "should work work with redirects when other outlet comes before the one being activated"

[atscott]

new global presubmit

[atscott]

@alxhub PTAL

The internal failure was due to a stack overflow caused by the switchMap approach (test added that failed in the previous version). The new approach is to

1. separate the Routes by their outlet names
2. run the exact same algorithm as before, but ignoring errors from expanding routes of outlets that differ from the one being activated (I wish the git diff was showing this better).
3. Wait for each of the observables-by-outlet to complete and return only the one for the outlet being activated

This has the added benefit of loading all matched outlets concurrently. Additionally, the concatAll, first() is able to exit the processing as soon as a match is found rather than proceed with the switchMaps and produce no-ops for following routes of the same outlet like the old algorithm would.

global presubmit

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[danylobilokha]

hi guys, any update on that? Stuck with this issue and need this PR

[atscott]

@danylobilokha it’s available in the v11 release, currently 11.0.0-next.2

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