loader: remove references to shlwapi.lib

loader: remove references to shlwapi.lib

This change improves loader compatibility with Universal
Windows drivers[1]: shlwapi.lib is not a part of allowed APIs.

It seems PathFileExists call was already redundant as
ERROR_MOD_NOT_FOUND was already returned by the loader in
cse of wrong dll path.

PathIsRelative is replaced by equivalent check.

[1] https://docs.microsoft.com/en-us/windows-hardware/drivers/develop/getting-started-with-universal-drivers

Change-Id: I36854f38078670ac033e8bd415dbf368391e8448