toolchain.List: don't search for nested toolchains

Searching for nested toolchains takes a long time because it means
traversing into toolchains' build/temp dirs. It's a reasonable
expectation that toolchains won't be nested. This speeds up `src make`
and `src makefile` from 15s to ~100ms on a repository with 50 source
units.
Merge pull request #95 from sourcegraph/no-nested-toolchains