Allow genrules to depend on tools with symlinks

Symlink PackagingSpecs don't have a sourcePath, so it crashes soong
if you try to add a tool with symlinks to a genrule. Make RuleBuilder
ignore symlink packaging specs.

We could make RuleBuilder add them to the sandbox later if needed.

Bug: 395988167
Test: go test
Change-Id: I4dd2bdec6ff6e94f5014889e4a9378ae4c20f9e3