use extension name for determining an extension is installed in the index

[wfjsw]

Rationale behind this change

• If users are using insteadOf git config directive, the entire installed tag will bork as no remote url will match that in the index.
• Even though with the situation where the remote url may be different for an already installed extension (forks), users are expected to fail later for AssertionError: Extension directory already exists
• The original implementation, however, does cover the case where an extension is installed with a non-default folder name. But most of these cases are unlikely to have git enabled (say "Download ZIP") so there will be no remote field anyway.

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[AUTOMATIC1111]

Maybe combine both? An extension is installed if either the remote matches or the directory name.

[wfjsw]

Maybe combine both? An extension is installed if either the remote matches or the directory name.

Sure. That could be better.