[Xamarin.Android.Build.Tasks] <GetImportedLibraries/> perf improvements

[jonathanpeppers]

I noticed a target running on an incremental build with only a XAML
code change:

116 ms  _BuildLibraryImportsCache                  1 calls

This was using VS 2019 GA, just a blank Xamarin.Forms app.

Reviewing <GetImportedLibraries/> it had some general perf
improvements that could be done:

• It called Directory.EnumerateFiles three times...
• It used a bunch of LINQ where a foreach would do.

Additionally, I saw some improvements I could do at the MSBuild target
level:

<Target Name="_BuildLibraryImportsCache"
        Inputs="$(MSBuildProjectFullPath);@(_ReferencePath);@(_ReferenceDependencyPaths);$(_AndroidBuildPropertiesCache);$(_AndroidLibraryProjectImportsCache)"
        Outputs="$(_AndroidLibraryImportsCache).stamp">

This target operates on the output of _ResolveLibraryProjectImports,
but it was re-running if any assembly changed?

I think the Inputs and Outputs would more appropriately be:

Inputs="$(_AndroidLibraryProjectImportsCache)"
Outputs="$(_AndroidStampDirectory)_BuildLibraryImportsCache.stamp"

This way, if _ResolveLibraryProjectImports runs and does work, it
writes to $(_AndroidLibraryProjectImportsCache). We then only need
to run this task when that file changes.

I also updated this target so it follows our new convention for
placing stamp files in $(_AndroidStampDirectory).

Results

An example of an initial build of the Xamarin.Forms project in this
repo:

Before:
299 ms  GetImportedLibraries                       1 calls
After:
172 ms  GetImportedLibraries                       1 calls

And then an incremental build, that is a XAML change:

Before:
109 ms  _BuildLibraryImportsCache                  1 calls
After:
n/a

The task is skipped now:

_BuildLibraryImportsCache:
Skipping target "_BuildLibraryImportsCache" because all output files are up-to-date with respect to the input files.

Overall, I would say there is a ~120ms improvement on initial build.
Incremental builds with XAML changes have a ~100ms improvement.

[jonathanpeppers]

Build logs: getimportedlibraries.zip

[dellis1972]

3 of the errors look like those weird ssl issues.

One is this

System.IO.FileNotFoundException: Could not find file "/Users/builder/jenkins/workspace/xamarin-android-pr-pipeline-release/xamarin-android/bin/TestRelease/TestResult-Mono.Android_TestsMultiDex.xml"

not sure it is related though,

[jonathanpeppers]

I restarted the macOS build -- if we just had network failures would be OK, but the multi-dex tests failing to start were concerning.

[jonathanpeppers]

I think the macOS build is OK:

They seem network-related, like the other PRs we have right now.