Optional xcframework support for Linux

[hassila]

Provide experimental support for XCFrameworks for Linux for well-defined, identical, deployments.

Motivation:

Providing informal, experimental support for (very) heterogenous cluster deployments on Linux as discussed previously in #5714

It allows for deployments which use XCFrameworks and library evolution on identically patches / toolchains on Linux as an interim escape hatch until wider support might be available.

Modifications:

Enable XCFrameworks for Linux if the _SWIFTPM_EXPERIMENTAL_LINUX_XCFRAMEWORK environment variable is set.

Result:

Users may experiment with library evolution and XCFrameworks on Linux without having to roll their own toolchains while waiting for wider support for shared libraries / evolution on Linux in general.

The default behavior remains unchanged and there should be zero impact for users who doesn't set this environment variable.

[MaxDesiatov]

To manage expectations on this, I personally don't think XCFrameworks should be ever supported anywhere outside of builds on macOS with the Xcode build system. It's an Xcode-specific format that's not meant to be cross-platform outside of Xcode use cases isolated to Darwin. The proper format for true cross-platform binary libraries is .artifactbundle, which was designed in the open specifically for that purpose.

Thus if you'd like to push forward with the overall concept of cross-platform binary libraries, I advise switching to .artifactbundle and improving upon #6967 with questions addressed that we've identified in #7035 (comment).

[hassila]

Thanks @MaxDesiatov, we'll have a look at that, appreciate the pointers.