Thanks for taking the time to help me fix this package stuff. So I wiped out my package.conf.d/ and tried to reinstall my stuff. Installing template-haskell 2.5.0.0 is ok (2.6 is not, fails to compile on ghc 7.0.3) but then:
D:\projets\>ghc --make -package ghc main
<command line>: cannot satisfy -package ghc
(use -v for more information)
That'd be expected; you shouldn't simply obliterate package.conf.d, you need to keep ghc's boot libraries (which include itself as a library). If the boot libraries are somehow hosed, you need to reinstall the compiler to get them back.
Although, I think you were headed there anyway: template-haskell is a boot library, since it's very tightly bound to the exact compiler version, so it's almost always wrong to install it in any way other than as part of installing GHC itself.