Cleanup THH function use from built module namespace

Currently, THH.hs "injects" into the built code names of library
functions like Text.JSON.makeObj, Ganeti.JSON.fromObj, etc. built
directly from strings, via (e.g.)

  varE (mkName "makeObj")

This means that the "makeObj" name must exist in the target module,
i.o.w. must be imported there. This leads to the strange case of
having to have imports that do not appear at all in the used
(template) code, but are needed to satisfy this "hidden" dependency;
look at Ganeti/Jobs.hs before this patch, for example.

This is also not very obvious, because we usually import Text.JSON
anyway; I only stumbled upon it while doing some cleanup work.

So to clean this up, the current patch changes the THH.hs to use not
string-derived, but identifier-derived names («'identifier» versus
«mkName "identifier"»); this is better, as the names must be
resolvable when compiling THH itself (once), and not when compiling
the multiple derived modules. As you can see, this allows removal of
extraneous imports from various modules.

Background information: an `mkName "foo"` results in a name of flavour
NameS (“An unqualified name; dynamically bound”) or alternatively to a
qualified name, but still dynamically bound. Whereas what we want is a
statically bound name: `'foo` results in a NameG flavour, “Global name
bound outside of the TH AST: An original name”.

One more explanation: the change is similar to going from 'x = eval
"map"' to 'x = map'; the name is no longer dynamically evaluated, but
statically when the module is compiled. In our case, previously names
were bound at target module compile time, now they are bound at THH.hs
compile time.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>