Installs libmimalloc.a into $PREFIX instead of into $PREFIX/lib

Plist:
```
cmake/mimalloc-config-version.cmake
cmake/mimalloc-config.cmake
cmake/mimalloc-%%CMAKE_BUILD_TYPE%%.cmake
cmake/mimalloc.cmake
include/mimalloc-new-delete.h
include/mimalloc-override.h
include/mimalloc.h
lib/libmimalloc.so
lib/libmimalloc.so.2.0
libmimalloc.a
mimalloc.o
```

Additionally, it installs a stray file ```mimalloc.o``` that needs to be removed.

Start by checking the install and packaging rules associated with the listed cmake/mimalloc*.cmake files and the Plist entries. Done means libmimalloc.a is listed under lib/ and the stray mimalloc.o entry is removed, while the existing headers and shared-library paths remain correct.

Written by the indexing model from the issue text.