uscan: add "compat" target to download components with a compatible version

Closes: #973814:

uscan offers some target for components: ignore, same,... same is strict and matches only the exact same version, while ignore doesn't check anything.

It could be interesting to have a compat target that accept to download the last component whose version is compatible (using Semver algorithm). Example: nodejs needs to embed a compatible @types/node nodejs module. Nodejs unstable version is 12 and @types/node does not follow any minor version of nodejs, so:

• same will often fail
• ignore will download @types/node version ≥ 14, corresponding to last published version. This generates build errors (see #964637 for example)

The compat target (with compat_group and compat_checksum) will avoid to modify debian/watch regexp and is more precise:

• if nodejs version is 12.2.3 and @types/nodejs has 12.2.3, 12.2.5, 12.5.6, compat will choose 12.2.5 (last where major and minor match)
• if nodejs version is 12.2.3 and @types/nodejs has 12.1.3, 12.3.3, 12.5.6, compat will choose 12.5.6 (last where major match)
• if nodejs version is 12.2.3 and @types/nodejs has no 12.x version, compat won't return any compatible version

Another example:

• a module has no types embedded in its version 1.x, @types/foo is available and compat downloads it
• upstream decide to publish 2.X with embedded types definitions. @types/foo won't publish any 2.x version. Then uscan fails and DD discovers that embedding @types/foo is no more needed (many examples in nodejs packages)