Simplify library search process

When searching for libraries we were looking for both the `lib`
prefixed version and the non-`lib`-prefixed name.

While refactoring code in #13799 I noticed that we really don't need
to be handling both these cases.  We can assume that file on disc will
always have the lib prefix and the library names being processed here
will never have it.

This does remove some undocumented/untested (and I would have better
not supported) uses.

1. Linking with `-llibc` rather than `-lc` will no longer work.
2. Linking a library called `foo.a` (without a `lib` prefix) via `-lfoo`
   will no longer work.

Neither of these use cases is supported by gcc or clang and I don't
think it was ever intentional that we supported these.

ChangeLog.md

@@ -230,6 +230,8 @@ See docs/process.md for more on how version tagging works.
 - In order to behave more like clang and gcc, emscripten no longer
   supports some nonstandard methods of library lookup (that worked
   unintentionally and were untested and not documented):
+- Refactoring of the library search algorithm means that emscripten no longer
+  supports some (unintented) methods of library lookup:
     1. Linking with `-llibc` rather than `-lc` will no longer work.
     2. Linking a library called `foo.a` via `-lfoo` will no longer work.
        (libraries found via `-l` have to start with `lib`)