In commit b2c14bc774 (cmake -E: Add cmake_transform_depfile internal
command, 2020-10-02, v3.20.0-rc1~684^2~2) a partial implementation for
converting MSBuild "TLog" format files was added. However, it is
unused and incomplete:
* Does not enforce absolute paths as TLog docs require.
* Does not upper-case paths as TLog docs recommend.
* The TLog format semantics are not equivalent to `out: in`.
Remove the unused TLog transform infrastructure to avoid confusion.
Issue: #20286
Most calls to `MaybeConvertToRelativePath` use one of our common work
directories (e.g. top of the build tree) as the local path. Add helpers
for each of the common cases to simplify and clarify call sites.
