support distribution metadata substitution in write_to path

[RonnyPfannschmidt]

trafficstars

unfortunately current packaging practices make it necessary to have the package name yet again in the src folder

practically this means people have to spell the package name in the write_to parameter

they should be able to do something like

[tool.setuptools_scm]
write_to = "src/{toplevel}/_version.py"

Originally suggested by @smarie in https://github.com/pypa/setuptools_scm/issues/582#issuecomment-862207084

ideally in future src will directly map to the toplevel of a package

[smarie]

So efficient as always. Thanks @RonnyPfannschmidt !