Add rootpath(s) and execpath(s) functions to template expansion

In addition to the $(location) function, we now also support $(rootpath) and
$(execpath) functions.

Unfortunately, we have to do this in two places since the Skylark API for expand_location has to continue calling into LocationExpander in order to preserve its semantic contract.

Progress on #2475.

RELNOTES[NEW]:
    In addition to $(location), Bazel now also supports $(rootpath) to obtain
    the root-relative path (i.e., for runfiles locations), and $(execpath) to
    obtain the exec path (i.e., for build-time locations)

PiperOrigin-RevId: 174454119

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