Actualize project root directories

This solves the problem of changing path spelling on platforms with case-
insensitive filesystems.

For example, you may build a project in the current working directory without
specifying any paths. This means the current working directory will be used as
the project's root. On Windows this could be C:\x.

Now you are building another project that imports the above project and you
specify config.import.x variable pointing to the above build. But you are lazy
to type capital C so you spell it as config.import.x=c:\x.

What happens now is the value from config.import.x is used as the project
root. And now it is a different spelling compared to your original build. This
is not a problem when the build system itself is concerned -- it is smart
enough to use case-insensitive comparison. However, we often use roots to
derive other things, say, -I options that we pass to compilers. And these
options are normally no longer treated as (case-insensitive) paths. If they
are hashed and the result stored in depdb, then we end up with rebuilds that
are triggered by changes from C:\ to c:\.

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