Further module system revamp (fix #659)

To import a module now use:

    # Import module.jq file:
    import "relative/path/to/module" as foo;

    # Use the module's defs as foo::<def-name>

To import a JSON file:

    # Read file.json:
    import "relative/path/to/file" as $foo;
    #
    # Use as $foo::foo

Using `-L` now drops the builtin library path and appends the requested
path to the empty array (or the result of an earlier `-L`).

Support for the `$JQ_LIBRARY_PATH` environment variable has been
removed.

bytecode.c

@@ -9,6 +9,7 @@
 #define NONE 0, 1
 #define CONSTANT OP_HAS_CONSTANT, 2
 #define VARIABLE (OP_HAS_VARIABLE | OP_HAS_BINDING), 3
+#define GLOBAL (OP_HAS_CONSTANT | OP_HAS_VARIABLE | OP_HAS_BINDING | OP_IS_CALL_PSEUDO), 4
 #define BRANCH OP_HAS_BRANCH, 2
 #define CFUNC (OP_HAS_CFUNC | OP_HAS_BINDING), 3
 #define UFUNC (OP_HAS_UFUNC | OP_HAS_BINDING | OP_IS_CALL_PSEUDO), 4