use `emake test` instead of `make` in default src_test
consolidate libdir determination in internal get_libdir function

Instead of using nearly the same code in three places. In addition, we
can just export a renamed version of this function for the public
get_libdir implementation in EAPI 6.
make sure to always create requested default_${phase} functions for EAPIs

Default functions weren't getting created in certain cases since the
related function names already existed so the negated __is_function
check would fail.