Revert logic in cacheKey calculation introduced in #1163

[art-divin]

Context

The way cacheKey was calculated before #1163 was that includedFiles collection was empty, whereas after #1163, that collection always contains file paths.
Because of the aforementioned change in the logic of calculating cacheKey hash, the following issue occurs:

1. buildDir path includes all files in .build folder, and thus, cacheKey depends on the build folder contents
2. buildDir contains files like master.swiftdeps, .swiftsourceinfo, main.swift.o, DWARF/SwiftTemplate, build.db, context.bin, which change every time a build is performed (I suppose timestamp is used in these files in one way or another).
3. Due to this change, hash of the key does not match and "previous" cache get deleted instead of being reused

Solution

Revert this change and advise to use --cacheDisabled flag because it would provide exactly the same behaviour as it is now in version 2.0.3.

Resolves #1196