'Unlink' the implementation of `--self-contained` from the MSBuild project property `SelfContained`

### Is your feature request related to a problem? Please describe.

`--self-contained` currently directly sets the value of the `SelfContained` MSBuild property, which can lead to nonsensical combinations of behavior.

For example, a `PublishAOT=true` application will _always_ be self-contained, and so `--self-contained false` should either error with a clear error, or mean nothing at all.

### Describe the solution you'd like

To do this, we need to separate the implementation detail of the `--self-contained` flag today - instead of directly setting `SelfContained`, which would be forcibly set due to MSBuild's global-property-from-command-line semantics, we should set a different, private property that we use to compute the final `SelfContained` property value. This would allow the SDK targets to deliver a targeted error experience _or_ silently address the problem by ignoring the `false` for cases like PublishAOT where the behavior is implicitly required.

This has become more of a problem since https://github.com/dotnet/sdk/pull/52333 fixed the behavior of `--self-contained` to always parse the boolean value the user provided - see https://github.com/dotnet/runtime/issues/127049 for an example of how users experience this.