whereami: Use SOURCE_DIR variable

[dg0yt]

Replace the hard-coded source dir location with an public source dir output variable provided by FetchContent. This decouples the SDK from the current implementation details of FetchContent, and it allows packagers or disconnected builds to inject a pre-fetched location via input variable FETCHCONTENT_SOURCE_DIR_WHEREAMI-EXTERNAL.

[CLAassistant]

All committers have signed the CLA.

[dg0yt]

Applied (by direct replacement) in https://github.com/microsoft/vcpkg/pull/40122.

[dg0yt]

Closing in favor of #100.