fix: stop busser plugin list printing git errors and a phantom plugin

[tas50]

fix: stop busser plugin list printing git errors and a phantom plugin

Running the command against a normal install produced this:

$ busser plugin list
fatal: not a git repository (or any of the parent directories): .git
Plugin  Version
dummy   0.9.1
bash    0.1.4

Two separate problems, both user visible on a command people reach for when
something is not working.

The git error. gem_from_path preferred loading a gemspec file over
asking RubyGems. Loading a gemspec evaluates it, and these gemspecs shell out to
git ls-files to build their file list -- so every installed plugin printed a
git failure from a directory that is not a checkout. It now asks
Gem::Specification.find_by_path first, which answers from the installed specs
without running anything, and only falls back to a working-tree gemspec when
that comes back empty, which is the local-development case the fallback was for.
The fallback also now checks the gemspec exists before loading it.

The phantom plugin. dummy is a fixture shipped inside busser for its own
tests, not something anyone installed. busser test already passes over it, so
listing it as installed was inconsistent as well as confusing.

After:

$ busser plugin list
Plugin  Version
bash    0.2.0

busser test dummy and busser plugin install dummy still work -- those name
the runner explicitly and do not go through the listing.