Move ProjectCache namespace out of experimental

[JanProvaznik]

Fixes #11575

Context

Reflect that the ProjectCache is no longer used experimentally.

Changes Made

Made a copy of each class in the non-experimental namespace.

Testing

Existing tests

Notes

After insertion needs adjustment in VS.

[maridematte]

In this case, why are we having a copy of each class still in experimental?

[JanProvaznik]

In this case, why are we having a copy of each class still in experimental?

In order not to break existing consumers of that API. The Experimental versions of these files would be removed some time in the future.

[JanProvaznik]

I redid the merge as in BuildParameters the wrong namespace was included.

[JanProvaznik]

this commit 2e1d6d2 breaks the tests because the tests rely at the same time on the sample (which has the version 16.11 with the old namespace) and on the current implementation (using old doesn't solve it trivially nor the new)

@rainersigwald do you have an idea how to resolve? can we unpin?

[JanProvaznik]

this needs more work, this approach does not work to maintain compatibility

[JanProvaznik]

redoing this from scratch