GitLab

This removes the redundancy between `RelaxDepsNone` and
`RelaxDepsSome []` by removing `RelaxDepsNone`.

This way we avoid the risk of subtle bugs that can occur if the same
semantic value can be expressed in a non-unique way.

A further step to normalise the type would be to turn `[RelaxedDep]`
into `Set RelaxedDep`, but there is no operation that would
significantly benefit from that yet.

[skip ci]

Use package root as data-file base path, not cwd

With cabal projects a package might depend on another inplace package in
the same project, so we need to add envoronment variables for each of
them when using new-run.

Using the current directory worked with single packages, but cabal
projects can use packages in different places.

Fixes #4639

Refactor `getProgramInvocationOutput{,AndErrors}`

Bring stack.yaml up to date

This reduces code-duplication by sharing common code-paths.

Specifically, when `progInvokePathEnv` support was added, this wasn't
updated in `getProgramInvocationOutputAndErrors`.

...where it really belongs

Use default flags only for new-* path

Add staticlib

Allow --config-file flag for new-build

Upgrade to 2.0 branch.

- cabal-install 2.0 new-build has different directory
  layout; everything is adjusted for that.
- Using hvr's PPA for Linux builds, and a custom
  ezyang build for OS X

[ci skip]

Fixes #4625.

Leave single invocation of ar

Make new-bench actually run the benchmarks

Based on new-test.

Also rename config field configArDoesNotSupportResponseFiles into
configUseResponseFiles

Newer versions of ar program support @file argument, which allows to
supply all object files, however numerous, in one call to ar, thus
reducing overhead of creating multiple ar processes where each process
must fully re-read output of the previous invocation.