fix(cli/tree): buggy path inheritance in tree for single-file packages

[deciduously]

The test at packages/cli/tests/tree/package_single_file_local_import.nu on branch repro/path-joining-tree demonstrates a path inheritance bug in the tree implementation. The path for libiconv is stored as "libiconv.tg.ts" instead of "./libiconv.tg.ts". When inherited from bzip2's context via referent.inherit(), the path becomes "packages/bzip2/libiconv.tg.ts" instead of the correct "packages/libiconv.tg.ts".