fix(router-store): remove circular depedency in serializers

[timdeschryver]

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• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #1902

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

To be honest, I have no idea why this was a circular dependency 😅.
Importing serializer via their specific file names (e.g. ./serializers/default_serializer) in router_store_module.ts resolved this issue. To be safe I removed the entry file ./serializers/index and pointed every import to the specific file.

Also renamed shared to base as it felt weird to import from /shared.

[ngrxbot]

Preview docs changes for b606431 at https://previews.ngrx.io/pr1904-b606431/

[brandonroberts]

Verified the fix against a local reproduction of the issue