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.

tests/modules/a.jq

@@ -1,2 +1,2 @@
-module a {version:1.7};
+module {version:1.7};
 def a: "a";

tests/modules/c/c.jq

@@ -1,12 +1,14 @@
-module c {whatever:null};
-import a as foo;
-import d as d {search:"./"};
-import d {search:"./"};
-import e as e {search:"./../lib/jq"};
-import f as f {search:"./../lib/jq"};
+module {whatever:null};
+import "a" as foo;
+import "d" as d {search:"./"};
+import "d" as d2{search:"./"};
+import "e" as e {search:"./../lib/jq"};
+import "f" as f {search:"./../lib/jq"};
+import "data" as $d;
 def a: 0;
 def c:
-  if meh != d::meh then error("import into namespace doesn't work")
+  if $d::d[0] != {this:"is a test",that:"is too"} then error("data import is busted")
+  elif d2::meh != d::meh then error("import twice doesn't work")
   elif foo::a != "a" then error("foo::a didn't work as expected")
   elif d::meh != "meh" then error("d::meh didn't work as expected")
   elif e::bah != "bah" then error("e::bah didn't work as expected")