Apt-get wait for lock release

In Debian terminology, when you run

apt-get source linux-image-3.19.0-trunk-amd64

(or the equivalent apt-get source linux), you're actually downloading and extracting the source package. This contains the upstream code (the kernel source code downloaded from kernel.org) and all the Debian packaging, including patches added to the kernel by the Debian kernel team.

When you run

apt-get install linux-source-3.19

you're actualling installing a binary package which happens to contain the source code of the Linux kernel with the Debian patches applied and none of the Debian packaging infrastructure.

The source package's name is just linux; apt-get source will convert any binary package name it is given into the corresponding source package name.

By the way, since experimental packages aren't upgraded automatically, you should make sure you've updated your copy of linux-source-3.19 and re-extracted it before comparing; the .dts file you're seeing in your diff was introduced in the latest update. The packages currently in the archive all contain this file.

The remaining differences are pretty much normal: as has been indicated in the comments, debian contains all the packaging and is only in the source package, .pc is used by quilt to keep track of the original files modified by patches, and is also only in the source package, and the .1 files are generated manpages, probably a side-effect of the kernel build, and therefore only appear in the binary package (but they shouldn't really be there).

The reference package is the source package, as obtained by apt-get source. This builds all the kernel binary packages, including linux-source-3.19 which you install with apt-get install. The latter is provided as a convenience for other packages which may need the kernel source; it's guaranteed to be in the same place all the time, unlike the source package which is just downloaded in the current directory at the time apt-get source is run.

As far as documentation goes, I'd follow the Debian documentation in the kernel handbook (section 4.5). Rebuilding the full Debian kernel as documented in section 4.2 which you linked to takes a very long time because it builds a number of variants.

1. The Packages.gz contains a Filename field with a value that probably looks something like pool/main/n/name-of-package/name-of-package_version_amd64.deb. This tells the package manager to look at that URL.
   For instance, if you have the following line in your sources.list(.d):
   deb https://some-domain.com/some-url some-distribution main
   You will have the following file: https://some-domain.com/some-url/dists/some-distribution/main/binary-amd64/Packages.gz
   That file will reference pool/main/n/name-of-package/name-of-package_version_amd64.deb, so your package manager will look at https://some-domain.com/some-url/pool/main/n/name-of-package/name-of-package_version_amd64.deb.

2. Sources.gz works the same way, but is optional. It works with apt source, which will fetch you the source if you wish to compile it yourself instead of using the pre-built binaries.
   This is mainly useful for system administrators who want to patch the exact version of the package used by a distribution, rather than contributors/curious people who would just go to the project's homepage and follow the build instructions there.

3. If a package is in the distribution's pool directory but not in Packages.gz, it is most probably in another Package.gz. Notice how you have several words in a typical sources.list line:
   deb http://archive.ubuntu.com/ubuntu precise main universe multiverse
main, universe and multiverse each have their own Packages.gz file. You may need to enable some, as debian for instance disables non-free by default. However, some packages still won't be available (not even in pool). You may choose to add untrusted repositories (such as PPAs on Ubuntu), package the missing program yourself (so you and other people can use their package manager to manage that program) or simply build and install it yourself (you should use the /usr/local prefix to avoid conflicts with your package manager).

4. When a new (version of a) package appears, the Release file for the distribution is updated. This allows apt update to know it has to re-download Packages.gz.