[lldb] Support Python imports relative the to the current file being sourced

Make it possible to use a relative path in command script import to the
location of the file being sourced. This allows the user to put Python
scripts next to LLDB command files and importing them without having to
specify an absolute path.

To enable this behavior pass `-c` to `command script import`. The
argument can only be used when sourcing the command from a file.

rdar://68310384

Differential revision: https://reviews.llvm.org/D89334