SourceLayout: esi/, take 1

SourceLayout: esi/, take 1

Moved src/ESI* files into src/esi/.
Renamed ESI source files from ESIFoo.{cc,cci,h} to Foo.{cc,cci,h}.

No true source code changes.
No functionality changes were intended.
No runtime tests with ESI performed.

TODO: I did not move the tests/ESIExpressions test into src/esi because
we need to fix the entire testing layout before we should start
moving individual tests: They currently depend on individual source
and object files in src/ and in tests/ and that kind of dependency
should not be multiplied but removed.

TODO: Move ESI types into the Esi namespace.