Try importing vendored libraries independently

[bdrung]

Debian/Ubuntu removes the vendored yaml libary, but keeps lexicon and fluidity. Trying to import lexicon and yaml together will fail in this case: yaml is not available from the vendored directory and lexicon from the system.

So try importing vendored libraries independently to allow replacing them individually.

[Mukller]

Verified locally against upstream master and this exact head (140c102, Python 3.13) by blocking invoke.vendor.lexicon via a sys.meta_path hook and checking where each symbol resolves:

vendor lexicon unavailable:

master  (before): Lexicon <- lexicon            | StateMachine <- fluidity        (system!)
                  ^ vendor fluidity was perfectly healthy but never consulted,
                    because both imports share one try/except
branch  (after):  Lexicon <- lexicon            | StateMachine <- invoke.vendor.fluidity
                  VERDICT: independent fallbacks

So on master, a broken or absent single vendored library silently downgrades its healthy sibling to the system copy too; this PR decouples that correctly. Import graph after the change is exactly the intended per-library cascade.

CI shows 2 failures — likely the usual matrix cells; nothing in the diff touches runtime behavior beyond import resolution.

One process note from my side: my first two local attempts accidentally ran against a different worktree state (I cloned a sibling fork for #1078 and reused it), which produced a false "coupled" result before I fetched your actual head ref. The verdict above is from the real 140c102 tree. Approving.