Is there any software (preferably open source) that can be used to build one's own street views for areas/countries that are not covered by Google?

The software is not open-source but both Mapillary and OpenStreetCam have very permissive licenses. I contribute to and use both services to improve OpenStreetMap.

Mapillary: https://www.mapillary.com/

OpenStreetCam: https://openstreetcam.org/

OpenStreetCam itself isn’t quite closed-source: it is on Github. But the app was built on top of Facebook tools, which many people will not want on their phones. See the notorious outstanding Github issue [0]. At least OSC’s hard dependency on Google Play Services appears to have been removed, though – last time I looked into installing Mapillary, it still would not run on a bare Android like LineageOS.

[0] https://github.com/openstreetcam/android/issues/8

Any photogrammetry software will do the trick. A free one has been shared here recently: https://alicevision.org/ .

You can highly customize the Google Maps javascript service to load and connect you custom panoramas hosted on your own server.

I believe you can publish your own images and have them available via Google Maps but I've never done it. This might be a decent starting point: https://www.google.com/streetview/contributors/

I've seen a project by OpenStreetMap to upload dashcam images, and a quick search gave me this: https://openstreetcam.org

It started as "OpenStreetView", changed names due to Google, and has had a few different owners but is a continuation of the original project you saw.

Halt and Catch Fire seasons 1 and 2.