Handle missing receipt on `brew install`.

For example if this is for a really old keg, keg where a user has
manually removed stuff or used `brew diy`.

ISSUE_TEMPLATE: tweak instructions.

Use `sudo` if parent path of `target` is not writable.

Be clearer about what's expected and where to file things.

Document all short flags.

Fix the 'export PATH' message in `link` for a keg-only formula

Commit 4cae6a72 introduced the message,
but it printed the wrong path, e.g. for `brew link sqlite`:

```
If you need to have this software first in your PATH instead consider running:
  echo 'export PATH="/usr/local/opt/sqlite:$PATH"' >> ~/.zshrc/bin
```

where `/bin` is appended at the end, but should be inserted before
`:$PATH`: `echo 'export PATH="/usr/local/opt/sqlite/bin:$PATH"' >> ~/.zshrc`.
This patch fixes that and updates a test to verify it.

keg: correctly remove alias symlinks.

Fixes #2427.

cask: remove pre_bug_report links

missing_formula: pillow lives in homebrew/science

Remove osmium from blacklisted formulas

Migrator: various rename fixes

Otherwise we could end up removing files unnecessarily.

This avoids needing to use `force: true` and still let's Homebrew do
what we want with our own taps.

ruby_requirement: fix path prepend

Migrate between different taps and delete relevant subdirectories if
they can be safely. Also, tweak some naming and use utility methods.

Use `oh1` for moving kegs to a new directory.

Move the checks for moving kegs to a new directory into the relevant
method (`move_to_new_directory`) instead. Also, delete duplicated
directories when we’ve confirmed they are definitely duplicated rather
than telling users to do so.

Add methods to determine if a migration is needed and perform it if so
(and no-op if not). Additionally, make `ARGV.force?` get passed as a
parameter so it can be overridden without requiring users to pass
`—force`.

Cleanup: Continue on error removing keg

Fixes #2355
Create unremovable_kegs instance var
Check cellar cleanup failure after full cleanup completes
Use module_function in Homebrew::Cleanup as we never instantiate the
class

formula: handle missing deps when formula missing.

formula: check installed_alias_path exists.

Fix handling of tap migrations to new cask names.

Include the pinned version in brew outdated output for pinned formulae

update-test: Use git fetch --tags --depth=1