Collect package information from a .gradle script

[pombredanne]

This is a package manifest of sorts, equivalent to a pom.xml
See https://docs.gradle.org/4.2.1/dsl/

[pombredanne]

See the PR in #2822
We still need support for kotlin build scripts too.

[pombredanne]

See also #2761

[pombredanne]

For a kotlin example, see https://github.com/oss-review-toolkit/ort/pull/5118/files#diff-33aaad2b3b1af4604de8953a12f27fd248b6d4ab52d1d26ec5aff807123d2715