PR linters

Add GitHub integration(s) to detect common problems with PRs:

• Included .eopkg files (#377).
• Included Makefile (#377).
• Files in the wrong directory (ie packages/* and packages/*/*) (#377).
• Use of patch < instead of -i (#377).
• Missing homepage in package.yml (#377).
• Package is not bumped once (as a warning) (#475).
• Initial release on inclusion != 1 (#475).
• Incorrect package component (depends on #370).
• Alphabetized build dependencies (pkgconfig,A-Z,a-z,0-9) (#695).
• The commit must not end with a ] (#889).
• Version numbers that are valid numbers must have quotations (#963).
• pspec_x86_64.xml is invalid (cannot be parsed).
• Licences are valid SPDX qualifiers (#888).
• Check if there is a metadata.xml file for font packages. Lint the file.
• Appstream metadata (see below)
• abi_used_libs for packages in system.base and system.devel must be part of system.base or system.devel (https://github.com/getsolus/packages/pull/891).
• Hardcoded python version (python3\.\d+) in package.yml.

To be refined further:

• Single package bump per commit.
• PR build order.
• Package layout.