FSEntry: Throw an Exception when trying to read non-existent file ...

FSEntry.lookup can return null, and it would lead to a SEGV
with the attribute check, so add the forgotten check first.

clarify build.d documentation

Enhancement of integration test for windows (#2917) ...

* Run unittest directories for Windows

* Refactoring in response to review comments

PackageManager: Make lookup lazy again ...

We want to avoid loading the whole cache whenever lookup is called,
as most invocations of dub should result in a call to lookup,
since dependency resolution is much rarer than building with an
existing dub.selections.json.

Clear an unprinted deprecation during dub build ...

This deprecation is currently not printed by DMD,
but shows up if '-de' is used while compiling.
This solution is a workaround, as a proper fix
would mean not using the function in the first place,
but that requires a more in-depth rework.

Move the I/O part of Project.loadSelections to PackageManager ...

By doing so we can remove a sizeable amount of duplication as well
as the last `Dub.loadPackage` override. The resulting API is not
ideal, for example it doesn't allow to distinguish between parsing
errors and non-existent file, but will have to do for now.

Don't call `getPackageIterator` from `loadGitPackage` ...

`getPackageIterator` API forces us to do a refresh, which loads all
packages on the filesystem. However, for `loadGitPackage`'s use case,
we actually can simplify the check quite a bit.

Doing so means that we can now avoid doing a scan for projects
that use SCM-only dependencies.

Test Plan: On master, running the test (without the fix) triggers:
```
core.exception.AssertError@source/dub/test/base.d(790): Trying to access poisoned path: /dub/user/packages/poison/1.0.0/poison/dub.json
```
While with the fix, the test passes.

Clear up two deprecations triggering in unittest ...

Those are trivially fixed by using the new way of doing things.

Remove redundant add-path shell script test ...

This was introduced in #2526, however it has been superseded
by 1acfe7b .
Since the unittest version does not require cleanup and is simpler
to run, we can remove the shell version.

Merge remote-tracking branch 'upstream/stable' ...

This brings in the OMF fix required for master's CI to be green.

Trivial: Remove some no-longer-needed imports ...

Now that we are properly mocking file removal in store,
those imports are not used as the function no longer
does IO directly.

Remove support for OMF, make MsCOFF be x86 ...

Now that OMF support is being removed, dmd-latest is broken in the CI.
Since DMD no longer supports it, and we are mostly concerned about
supporting the latest 10 releases, we're keeping x86_omf and x86_mscoff
available but making them aliases to x86.

Merge pull request #2913 from ibuclaw/merge_stable ...

merge stable

Merge remote-tracking branch 'upstream/stable' into merge_stable

update version to v1.38.0

Fix regression in module-declarations parser

Merge pull request #2909 from ibuclaw/merge_stable ...

update version to v1.38.0-rc.1

Merge remote-tracking branch 'upstream/stable' into merge_stable

update version to v1.38.0-rc.1

Merge pull request #2908 from dlang/refactor_getpackageconfigs ...

Refactor getPackageConfigs

Move the code for collecting package information to a separate method.

Reorder code so that local functions come before the high-level algorithm.

Consolidate package related information in getPackageConfigs into a single struct.

Remove unused function.

Refactor removeConfig with filterInPlace.

Add filterInPlace utility function. ...

Note: this function should probably be added to Phobos eventually.

Fix parameter naming convention.

Merge pull request #2905 from dlang/speed_up_config_resolution ...

Speed up configuration resolution

Merge pull request #2907 from ibuclaw/merge_stable ...

merge stable

Merge remote-tracking branch 'upstream/stable' into merge_stable