Do not register missing deps

This is a fix to aosp/2260763 and skips the existence check corectly.
Previously, it would not throw an exception during Soong analysis, but
would add the path to a `MissingDeps` object. The ninja graph generated
using this would throw an exception during ninja execution.

We should not do this check for `cc_api_library`. The prebuilt
library.so is missing in the build graph of the inner tree (expected),
but it will be present when the orchestrator creates the combined
multi-tree ninja graph.

Test: rm -rf out && multitree_build vendor/vendorimage
(I did not clean out/ in aosp/2260763, and therefore did not catch this
earlier)
Test: go test ./cc

Change-Id: I68d245acae3bfb777bfc8a72fb7cd4909cb0a289