build_path ignored

### The problem

esphome core config is no longer honouring the 'build_path' option exactly, it seems to be prepending it with './.esphome/'

I hadn't updated in quite some time but this used to work, and definitely doesn't work in 2023.9.x and 2023.10.3

using, eg ,
```
esphome:
build_path: ./out/project0
```
code used to be generated as stated, but now it goes into './.esphome/out/project0'

### Which version of ESPHome has the issue?

2023.10.3

### What type of installation are you using?

pip

### Which version of Home Assistant has the issue?

_No response_

### What platform are you using?

ESP32

### Board

_No response_

### Component causing the issue

_No response_

### Example YAML snippet

```yaml
esphome:
name: project0
build_path: ./out/project0
```

### Anything in the logs that might be useful for us?

_No response_

### Additional information

_No response_

Reproduce the issue with ESPHome 2023.10.3 and the YAML example from the report, then trace how the esphome.build_path value is resolved during code generation. Compare the configured path with the generated output location; done means ./out/project0 is used directly rather than being placed under ./.esphome/.

Written by the indexing model from the issue text.