build: move oss required to build conhost out of dep/ (#5451)

This change is necessary as the dep/ folder is not synced into the
Windows source tree.

I've also added a build rule producing a lib for {fmt}.

This will be required for our next OS ingestion.

.github/actions/spell-check/excludes.txt

@@ -52,6 +52,7 @@ SUMS$
 \.yml$
 \.zip$
 ^dep/
+^oss/
 ^doc/reference/UTF8-torture-test\.txt$
 ^src/interactivity/onecore/BgfxEngine\.
 ^src/renderer/wddmcon/WddmConRenderer\.

.github/actions/spell-check/whitelist/whitelist.txt

@@ -1656,6 +1656,7 @@ osfhandle
 OSG
 OSGENG
 osign
+oss
 ostream
 ostringstream
 Ou