Returns the path on the local filesystem to the root of
null if the root cannot be found.
Note: this assumes a pub-style package layout. In particular:
If a package root is being used, this assumes that it contains symlinks to packages' lib/ directories.
If a package config is being used, this assumes that each entry points to a package's lib/ directory.
If these assumptions are broken, this may return
null or it may return
an invalid result.
null if the package root is not a
file: URI, or if the package
config entry for
package is not a
String packagePath(String package);