Since commit v3.9.0-rc1~309^2 (include_external_msproject: Honor
MAP_IMPORTED_CONFIG_<CONFIG>, 2017-04-04) we accidentally honor
`MAP_IMPORTED_CONFIG_<CONFIG>` while generating the `.sln` file entries
for normal targets.  This causes `devenv.com`-driven builds to use the
mapping incorrectly for normal targets.  Check that a target really
comes from `include_external_msproject` before considering the map.

Furthermore, when we do use the map, we should only take the first entry
if more than one configuration is specified.  Otherwise we end up giving
VS a configuration name with a `;` in it.

Fixes: #17276