[NFC] Rename `SPMTestSupport` to `_InternalTestSupport`

[MaxDesiatov]

There's no such product or entity as "SPM", this contraction is just as invalid as "MOS" would be for macOS, or "SNIO" for SwiftNIO. All marketing and documentation materials consistently refer to it as "SwiftPM", and we should stay consistent in our codebase too. This also simplifies code search, as anything named as "SPM" almost never shows up in results when searching globally for "SwiftPM".

We're not using SwiftPMTestSupport name here, as it might mislead contributors into thinking this module is somehow related to the swift test command. This module is purely internal, used only in modules with unit tests for SwiftPM itself, and is not supposed to be used by any packages depending on SwiftPM. Thus all @_spi declarations in it were also changed to package.

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test linux