Installing a board package via Board Manager completely screwed up another unrelated one

I have Arduino 1.8.7 and I had installed the **esp8266 boards** from https://github.com/esp8266/Arduino
I had installed them from the GIT repositiry directly into `[arduino folder]/hardware`, NOT the Board manager.
Everything was working fine and I had a sketch that I could compile for an ESP8266 board and upload to the board without any issues.

Then I installed the **ESP32** cores from Espressif (https://github.com/espressif/arduino-esp32) using the **Board Manager**.
I didn't even try to use them. I intended to use them later.

So, I opened my sketch for my ESP8266 board, and I kept the ESP8266 board selected. I tried to compile and upload it to the board as I had done hundred of times (I hadn't modified the sketch, I hadn't updated the ESP8266 cores, nothing had changed besides installing the completely unrelated ESP32 boards that I was not using in any way).

When trying to compile, I would get a nonsense error message related to the ESP32 boards, mentioning a file in the ESP32 package, despite the fact that I was trying to compile a sketch for a ESP8266 board:

```
Error compiling for board SparkFun ESP8266 Thing Dev.
fork/exec /home/teo/.arduino15/packages/esp32/tools/esptool/2.3.1/esptool: no such file or directory
Error compiling for board SparkFun ESP8266 Thing Dev.
```

This is absolute nonsense.

I reported this to Espressif (https://github.com/espressif/arduino-esp32/issues/2137), but their response was:
> the esp32 board package installs nothing outside of the .arduino15/packages/esp32 directory. Any interference with another board's core files is entirely an arduino IDE issue.

So, this seems to be a bug in the Arduino IDE itself.

Additionally, when I removed the ESP32 boards via the Board Manager, I got an error:


though the boards do appear to have been removed and the sketch for the unrelated ESP8266 board now does compile as expected.

Start by reproducing the installation sequence in Arduino IDE 1.8.7: install ESP8266 under [arduino folder]/hardware, then install ESP32 through Board Manager. Compare the ESP32 path under .arduino15/packages/esp32 and the esptool error with the selected ESP8266 board. Done means ESP8266 sketches still compile and upload, and removing ESP32 does not produce the reported error.

Written by the indexing model from the issue text.