Use solc --base-path to project root by default

[mattsse]

currently, source files in the compilerinput are canonic, this can cause some issues with metadata/hashes.

in #1307 we already save the artifacts with relative files, this should be obsolete if we set base-path == root and strip the root from all sources

[mattsse]

there's actually no support for setting the basePath in the compilerinput, instead we should be able to get the same behaviour by setting current dir when invoking solc to root and stripping the root from the compilerinput sources

or similar to how remappings are provided in standard-json input mode