Backport of #2673 and #2855 to v7.
See those two PRs for detailed analyses.

CC: @ViktorHofer @AArnott @jacekmlynek
Since you've previously been involved in package vs framework references.

Additional details:

In #2122 (comment) I experimented with referencing 6.9.0 and 6.10.0 as nuget packages from a net47 project and then comparing detailed MSBuild logs.
It showed how 6.10.0 no longer had conflicts between package and framework assemblies.

Doing that same comparison between main and this PR, the MSBuild log is now down from 90 to 0 occurrences of

Encountered conflict between 'Reference:'System.Foo' and 'Platform:System.Foo'

and now skips the running the build task AddFacadesToReferences

Comparing the bin/ folder of a net47 project that references Fluent Assertions via a nuget packages.
main:

PR:

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
• The code complies with the Coding Guidelines for C#.
• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
• If the PR adds a feature or fixes a bug, please update the release notes with a functional description that explains what the change means to consumers of this library, which are published on the website.
• If the PR changes the public API the changes needs to be included by running AcceptApiChanges.ps1 or AcceptApiChanges.sh.
• If the PR affects the documentation, please include your changes in this pull request so the documentation will appear on the website.
  • Please also run ./build.sh --target spellcheck or .\build.ps1 --target spellcheck before pushing and check the good outcome