Fix dependency inference handling of dependencies on self

[Eric-Arellano]

Before:

▶ ./pants dependencies src/python/pants/util:util
3rdparty/python:ansicolors
3rdparty/python:dataclasses
3rdparty/python:typing-extensions
src/python/pants/util/contextutil.py
src/python/pants/util/dirutil.py
src/python/pants/util/meta.py
src/python/pants/util/osutil.py
src/python/pants/util/strutil.py
src/python/pants/util/tarutil.py

After:

▶ ./pants dependencies src/python/pants/util:util
3rdparty/python:ansicolors
3rdparty/python:dataclasses
3rdparty/python:typing-extensions

We always intended for this behavior, but this regressed once we added generated subtargets.

[ci skip-rust-tests]

[stuhood]

Deja vu?

[Eric-Arellano]

Hehe. CI failure on the other one that a GitHub URL won't work. Restarting the build and pulling didn't help, so I figured I'd try this.