Add support for patches

[mpilgrem]

Based on a discussion in the Cabal and Hackage room on Matrix, initiated by @juhp:

In short, the ability to extend a project description to specify one or more *.patch files in respect of a package, for one or more packages, the patch files being applied in the order listed to modify the source code files before they are built; and the extension either happening at the command line or by agumenting Stack's project-level configuration file.

---

Example of a *.patch file:

• https://github.com/haskell/haskell-language-server/commit/fb17921128bd56ba74872cae9539767e63b9fd79.patch

Example of what @juhp did, using RPM macros:

cabal unpack %{ghcide}
sed -e "s/@GHCIDE@/%{ghcide}/" %{SOURCE1} > cabal.project
( cd %{ghcide}
  %patch -P1 -p2
)

Example of hypothetical command line options:

stack build --patch ghcide:the-first.patch --patch ghcide:second.patch
-  
stack build --patches "ghcide:first.patch,second.patch ..."

[mpilgrem]

Observation:

A project-level configuration file can specify an extra dep by reference to (1) a Git or Mercurial repository as at a specific commit; or (2) a local file path:

• https://docs.haskellstack.org/en/stable/topics/package_location/#git-and-mercurial-repositories
• https://docs.haskellstack.org/en/stable/configure/yaml/project/#extra-deps

An extra-dep shadows a snapshot.

[mpilgrem]

Complication:

Fundamental to Stack is the classification of a package (identified by a name and version and, if applicable, Cabal file revision number) as either immutable or mutable. Packages deemed immutable are assumed by Stack to be unchanging; once built, Stack caches the products of building. In what sense, if any, can you 'patch' (change) an immutable package? Packages that are mutable are:

• project packages;
• extra-deps identified by a local file path; and
• packages that depend, directly or indirectly, on a mutable package.