proposal: support indirect dependency

[MeteorsLiu]

We found libxml2 requires zlib indirectly, which means it requires zlib in code but not in header.

In that case, llcppg cannot recognize this indirect dependency, which will cause the failure of llpkg dependency analyzing.

So i proposal to add a blank module path to solve indirect dependency.

Example:
libxml2_autogen_link.go:

import _ "github.com/goplus/llpkg/zlib"

llcppg.cfg:

{
  ...
  deps: [..., "github.com/goplus/llpkg/zlib@v1.0.0"]
}

[luoliwoshang]

without this feature,we maybe can't analyze the indirect dependency and then fetch it's indirect dependency's binaries (current will remove dependency which is decl in deps but not indirect use in header file),lot's of package have this situation goplus/llpkg#43 goplus/llpkg#9 goplus/llpkg#30 and we need run these package normally in llgo goplus/llgo#1058