Hide KtTestCompiler as internal

[arturbosch]

Expected Behavior

KtTestCompiler is an implementation detail.
It should be made internal.
Users should use compileContentForTest
createEnvironment and createPsiFactory should be accessible as top level functions.
setupKotlinEnvironment should be prefered.

Observed Behavior

KtTestCompiler is used directly.

Steps to Reproduce

1. Try using KtTestCompiler directly in a testcase.
2. KtCompiler cannot be found as it is from an implementation dependency of detekt-parser.

Context

Testing custom rules.
#2720 (comment)