Rework download area support (#908)

Closes #904
* Make all data-related targets like `download*`, `import-osm`, `import-borders`, and `generate-tiles` into `area`-aware -- making it possible for multiple data files to coexist inside the `./data` dir.
* Add `make download area=... [url=...]` command to automatically download any kind of area by checking Geofabrik, BBBike, and OSM.fr, optionally from a custom URL. Supports `area=planet` too.
* Do not re-download area with `make download-*` if it already exists.
* Automatically rename `<area>-latest.osm.pbf` into `<area>.osm.pbf`
* If `area=...` parameter is not given to `make`, see if there is exactly one `*.osm.pbf` file, and if so, use `*` as the `area`.
* Configure many variables in the .env file, overriding the defaults in tools
* If `<area>.osm.pbf` exists, but `<area>.dc-config.pbf` is missing, generate it using `download-osm make-dc` command.

Also:
* closes #614
* closes #647
* partially addresses #261

.github/workflows/tests.yml

@@ -173,7 +173,7 @@ jobs:
           create_db() {
             make clean
             make init-dirs
-            cp ../ci_cache/perf-test-areas-latest.osm.pbf data/perf-test-areas-latest.osm.pbf
+            cp ../ci_cache/perf-test-areas-latest.osm.pbf data/perf-test-areas.osm.pbf
             make destroy-db
             make all
             make start-db