MpsLoadTask: simplify and fix the MPS home check

[sergej-koscejev]

The original code was checking getPath(), which is the full path of a file, against "build.txt", which is just the name.

Also, it isn't necessary to iterate through all the files when we can simply check for existence of a file.

[sergej-koscejev]

This went unnoticed for quite some time probably because it only works if assertions are enabled, and they usually aren't.

[ashatalin]

An issue in the MPS YouTrack: https://youtrack.jetbrains.com/issue/MPS-37669/pull-request-Modify-MpsLoadTask-to-correct-MPS-home-verification-logic

[sergej-koscejev]

Done by @mvlassiev