Import: share a referenced project's CCU instead of pickling it

[auduchinok]

A project reference pickles its signature once and every consumer unpickles its own copy: five projects
referencing one hub hold five copies of its signature TAST. The pickled tree is already in the shape
consumers need, so offer it directly through IImportedProjectCcu, and pickle only when something asks.
Depends on the shared imported assemblies change (#20296), whose keys let a consumer establish it will hold
the same ccus.

Four things make it usable:

• Remapped against a ccu made for this view, not the project's live one, which a consumer would else hold
  whole.
• PruneExportedSignatureInPlace brings it to the shape unpickling produces: no value definitions, no
  display-only data, no compiled-representation cache. Rescoping happens in the same traversal that builds
  the tree - ILScopeRef.Local is what internal is rooted at, so a consumer would otherwise read the
  reference's internals as its own.
• Every non-local reference is re-pointed at the reading project's ccu of the same name, as u_ccuref does
  for a consumer reading the bytes; a name the reader lacks keeps ours. This is what lets a consumer on
  another framework take the contents.
• What each name bound to is recorded, and a second consumer takes that copy only where it resolves all of
  them alike. Consumers that disagree get a copy each, capped at eight.

Binding runs once the batch is registered and before anything relinks: an assembly unpickled beside it
resolves the names it mentions against what is registered, and a delayed CCU is an error there rather than
a wait.

Solution	Projects	main	This	Retained
ReSharper.FSharp	10	385.94 MB	304.64 MB	-81.30 MB (-21.1%)
Fantomas	8	350.42 MB	275.79 MB	-74.63 MB (-21.3%)
FSharp.Compiler.Service	14	879.43 MB	818.39 MB	-61.04 MB (-6.9%)
FsToolkit.ErrorHandling	8	87.77 MB	69.36 MB	-18.42 MB (-21.0%)
Oxpecker	16	131.67 MB	114.24 MB	-17.43 MB (-13.2%)
Prime	5	112.22 MB	97.93 MB	-14.29 MB (-12.7%)
IcedTasks	7	96.97 MB	92.37 MB	-4.61 MB (-4.8%)
consoleapp	1	29.38 MB	29.37 MB	0.00 MB

The numbers are smaller due to other FSharpChecker creation flags.

consoleapp has no project references. Not all of this is the handover: switching the handover alone off and
on inside one build moves the compiler's own solution by 2 MB rather than 61, the rest coming from dropping
the language version out of the import reuse key, which lets projects that differ only in it share one
imported graph.

Also fixes a check-then-act race in BackgroundCompiler that predates this: builders were cached without a
second look under the gate, so callers arriving together each built the project, and a project built twice
is two of everything it imports. Nothing here survives that.

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md