Clarify what it means to be "unsupported" by a supports expression.

[BillyONeal]

@JavierMatosD Hopefully this makes it clearer for the customer you were working with.

Also fixes a few places where we were printing a spec but should have only printed a package name.

[JavierMatosD]

Thank you!

[dg0yt]

What I find confusing is how the message prints the requested platform first, i.e.:

atlmfc[core]:x64-linux is only supported on 'windows'

... this reads like "The linux package is only supported on windows".
Not sure how to improve this concisely, maybe

atlmfc[core] is unsupported on x64-linux but supported on 'windows'.
...

This area also needs improvements when a dependency is unsupported. ATM it doesn't tell the requested feature which asks for unsupported dependency. This is difficult to resolve for users. Example:

$ ./vcpkg install gdal[*]:x64-linux
Computing installation plan...
atlmfc[core]:x64-linux is only supported on 'windows'

[BillyONeal]

What I find confusing is how the message prints the requested platform first, i.e.:

That's fair, I'll take a look at this again

This area also needs improvements when a dependency is unsupported.

I agree.

[Neumann-A]

Suggested change

	"_UnsupportedSupportsExpression.comment": "An example of {spec} is zlib:x64-windows. An example of {supports_expression} is windows & !static.",
	"_UnsupportedSupportsExpression.comment": "An example of {spec} is zlib:x64-windows. An example of {supports_expression} is 'windows & !static'.",

[BillyONeal]

This (and all other examples) are generated code.

[Neumann-A]

Suggested change

	"_UnsupportedSupportsExpressionWarning.comment": "An example of {spec} is zlib:x64-windows. An example of {supports_expression} is windows & !static.",
	"_UnsupportedSupportsExpressionWarning.comment": "An example of {spec} is zlib:x64-windows. An example of {supports_expression} is 'windows & !static'.",

[ras0219-msft]

Suggested change

	"UnsupportedSupportsExpression": "{spec} is only supported on '{supports_expression}'. This usually means that there are known build failures, or runtime problems, when building other platforms. To ignore this and attempt to build {spec} anyway, rerun vcpkg with `--allow-unsupported`.",
	"UnsupportedSupportsExpression": "{package_name} is only supported on '{supports_expression}'. This usually means that there are known build failures, or runtime problems, when building other platforms. To ignore this and attempt to build {package_name} anyway, rerun vcpkg with `--allow-unsupported`.",

{spec} implies triplet information (see comment). I think {package_name} is the closest we have without defining a new placeholder.

[BillyONeal]

We are printing triplet information here; that's @dg0yt's comment :)

[dg0yt]

Package, features and triplets done right...
See the gdal example I added to #879 (comment).
What I want is app.

$ ./vcpkg install gdal[*]:x64-linux
Computing installation plan...
gdal[aws-ec2-windows]:x64-linux cannot be installed:
atlmfc[core] is only supported on 'windows'

[BillyONeal]

I agree that would be an improvement but that's more than the 10 minute "this should explain what we mean" fix I was going for here.

[BillyONeal]